There are so many variables to fuel economy that they simply can't investigate everyone that complains; they have to do some filtering to ensure that only legitimate cases get back to the service bay. I recall when the 5th gens came out, there were a few people complaining about their winter mileage not meeting the highway rating... turns out, they were letting the car idle for ~10 minutes to get the cabin up to temp before moving an inch. Yes, they were driving highway conditions after warming up, but that idling put them in a huge hole from the onset. Now, I'm not saying you are doing that, but there are definitely people out there that don't connect those cause and effect dots and the dealers do have the deal with those people as well.

Additionally, selling high mileage vehicles like the Prius means that you have a large number of customers that buy a vehicle for the fuel economy and when that magic 50mpg isn't met on the display, they freak out and head to the dealer. For example, say your driving conditions put you 10% below the combined rating. On a 4Runner, that turns out to be 17.1mpg instead of 19mpg. On a Prius, that is 45mpg instead of 50mpg. Despite being the same percentage off, numerically, it looks like the Prius is missing the target far more than the 4Runner is. I see people on the Prius boards annoyed that their Prius v is getting 42mpg instead of the 44mpg city rating even though that difference amounts to ~15 gallons of gas over a whole year. Nevermind the fact that the EPA test is very imperfect... (especially for hybrid city ratings... physics dictates that stop and go driving is very inefficient... it is just slightly less efficient in a hybrid).