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I'd say the manufacturers who are wiling to bribe them with the largest sum will get a biased inflated rating on KBB. Having an entity like KBB telling consumers what their vehicles are worth is like predicing the weather. You'd be better off looking at the sold auctions from EBay for a more accurate prediction because at least on EBay what the consumer is willing to pay determines the rate.
Although not perfect Kelly Blue Book is the most widely used resource at least here in the U.S. I'm not sure if manufacturers bribe KBB? I mean look at Mazda's latest cars. Small company yet with high resale values. Look at big bad VW or Ford with deep pockets. A lot of their cars have poor resale value. German cars in general have less resale value. Again it goes back to reliability and cost of ownership of a used car imo. The cost of maintenance and repairs. German cars are generally a lot more complex to fix over time. Japanese mainstream and even luxury cars are simpler and are more reliable hence the stronger resale values. Reliability and cost of ownership drives consumer's decisions. To me KBB is merely reflecting consumer trends. Take Hyundia/Kia. 20 years ago their resale values suck. Now I'd assume their better because their cars improved and the public was willing to pay more for their used cars. KBB was there to merely track the trends.
Furthermore say you'e in court of law involving the value of a car. The judge will refer to the blue book. People selling cars on Ebay or Craigslist for that matter would probably mention KBB as in "selling for $1000 below the blue book value".
Here's Ebay's take: "Being familiar with the blue book value is the first step to understanding the worth of a particular vehicle. Since Kelley Blue Book is an industry standard, it is also handy to have a copy, especially if one is planning to buy a vehicle or sell an old one." So even Ebay reminds its sellers/buyers to utilize the KBB as an "industry standard".