If it's an instant reading, you'll see it jump between 3 and 99.9 MPG with a slight adjustment of the accelerator! However if there's a trip MPG reading (average since you reset it) these are more accurate, but tend to be 2 to 4 MPG higher than actual. The longer you don't reset it though, the more accurate it tends to be.
The dealer is correct. Those readouts are not at all accurate. They can be a good reminder to drive efficiently, many can reliably tell you if you did better than your last tank, but they cannot be used to compare to other drivers/cars in a meaningful way.
I've done 3 fillups in my '14 Forester so far and the in-dash display has always been a little high. On my first tank, the display was reading 28.8mpg and actual was 28.4 or 28.5, I forget. The one I did the other day, the in-dash readout was 30.0mpg and actual was 29.5. So, they're in the ballpark, but no super accurate. On my 2012 Mustang GT, the in-dash display was actually about .75mpg lower than actual most of the time, I was was disappointed to find out Subie's read a little high. Either way, I'll be happy if I can get my combined mpg at 30 or better. So far, I'm about about 28.6 avg over 3 tanks.