You are only expected to be getting 310 ft-lbs of torque at the gear they dynoed the car in, probably 4th gear or 3rd gear. At 1st and 2nd gear, with the turbo at full boost, you are going to get more than 310 ft-lbs of torque. From what I've read on here, at 3rd gear and above, you are getting the promised 310 ft-lbs of torque from the engine. So not necessarily false advertising.
There is a reason why the Mazdaspeed community always tells new owners to not boost below 3000 RPM so as not to blow the engine. The SkyActiv 2.5T produces peak torque at lower than 3000 RPM, even more reason to be a little cautious in 1st and 2nd gear.
Edit: Actually I have it backwards, we are not supposed to be at full boost at low RPM on higher gears (5th or 6th). Since the CX-5 is automatic only, this shouldn't be a problem since the transmission will shift to a lower gear when needed.. I'm leaving my comment as the first paragraph is still valid, but I have it wrong on the second paragraph.