Hmm, well, Ive had some astonishing conversations with dealers over brakes and having spent 17 years in the Research and Development Dept of Ferodo - one of the major friction material manufacturers in the world, I can usually put up a pretty good fight.
Lets discuss your question first. There is no way of declaring what the life of friction material should or could be. It can range from indefinite at the top end to surprisingly short at the bottom. There are lots of variables but the main driver is temperature. Brake pads are made up from about 60-70% steel fibre. This provides physical strength to hold it all together and withstand the tremendous shear force involved with braking. The rest is made up from the abrasives and lubricants that stabilise friction through a wide temperature range and the resin system that binds it all together. The resin system degrades with temperature and constantly presents the friction element to the disc. Here is the crux of the matter. If you dont get the pads warm, you wont wear them out. If you get the pads very hot, they will wear out very quickly - its all down to duty level and it is on a non-linear scale so if you get a certain level of wear at a certain temperature, you might triple or quadruple it by doubling the temperature.
In a practical example, a very steady driver that lives in a flat location or only ever drives on a highway may rarely get the pads hot. This will lead to very long pad life. Somebody who drives a car hard, fully loaded with cargo and in the hills might get only 5000 miles or less. Pads react to the way they are used so getting them hot will excite friction and wear the resin which compounds the wear rate and pads that stay cool will go to sleep and reduce in friction - they get conditioned to the duty they are subjected to. Thats how temperature effects wear.
Now consider what happens to the vehicle. If you take an average duty cycle on a vehicle with varying duty and varying speeds and therefor varying temperature you can arrive at a suitable specification for the brakes. The rate of deceleration is measured as a percentage of 1g. 1g = 9.81 metres per second squared. Its about 32 ft per second squared. In practical terms, easing up at traffic lights might be about 30%g, a harsh stop where loose items would roll off a seat is about 50%g and a hard emergency stop will be about the point where the tires lose grip with the road surface and will be up at 75%g on a dry road. Using statistics, designers will calculate the amount of weight transfer during and average stop. This tends to be about 65% of the work done at the front and the remaining 35% at the rear and that is typically how big the pads are by surface area. That means that under average conditions, the pads will wear evenly despite the large difference in area. Are you with me so far?!!!
If the pads wear unevenly, in other words the fronts wear out before the rears or vice versa, there is something interfering with that predicted typical duty cycle. If the car is predominantly loaded and driven hard, it will transfer more weight - especially if there is a high centre of gravity - maybe a roof box and four or five passengers. If the car is driven normally with a loaded cargo floor or heavy weight in the back, the load apportioning valves in the hydraulic system will apply lots of pressure to the rear and the rears may wear out quickly.
Now for my prediction on what happened when your service dept hit you with this unexpected news. Firstly, and I say this with good and bad intentions towards the dealer, anything to do with brakes on a car is an easy money spinner for dealers. If they follow the manufacturers recommendations, the brakes should be in near to perfect condition with plenty of pad life and very little disc wear. The trouble is that they are designed wearing parts so that goes out the window soon after you start racking up the miles. The dealer is on a bit of a sticky situation because if the car has an accident after an inspection that can be in any way apportioned to the brakes, he is liable. If a mechanic inspects the brakes on a vehicle, he usually does it with the wheels on and that presents a problem because he cant easily get square on to the pads to get a good view - there simply isnt enough room. A good light and a mirror on a stick helps but still not ideal so he ends up taking a considered guess as much as an accurate measurement. Ill give you a good example of this. When I booked my 2016.5 in for service, they wanted to charge me for an oil change and a brake inspection. I wasnt having any of this as I do my own oil change and I am reasonably well qualified (read that as as expert as any brake engineer in the world) to measure my own pads. I told them that I would be changing the pads during my own inspection before the service. When I did this inspection, the pads were a lot less worn than I expected and I calculated they would do at least another 12 months so serviced the brakes and put the pads back. When I collected the car, they listed all the items that justified the bill and one of them was weve inspected the pads and are documenting them as 20% worn", the minimum the computer would allow them to enter. This was nonsense - they were approaching half worn but I didnt engage them, I just made a mental note to ignore any future life predictions.
Where does this leave you? Not an easy one. If my TV breaks I have to rely on an electronics expert to mend it. As far as Im concerned its a box full of magic and they could tell me anything, I would just have to trust them and cough up. I have to say that Im as surprised as you that the rears are so worn if they really are but its not impossible for all those reasons above and I cant get my head in from over here so Im not much use in that respect. I think I would go to a reputable local garage and ask them for a second opinion. If you do give in to the dealer, ask to see the new pads and ask them to keep the old ones and let them explain the difference in wear if it really exists.
