The MPG value that is shown next to my car (24.9) is actually incorrect. When you click on my 530i, it will show you that the MPG is 25.1, not 24.9.
I suspect the 24.9 is the average MPG for the two cars that I have under my account, but it should not be showing up like that when someone is just browsing for MPG results of BMW 530i vehicles.
Once in a while the average MPG while browsing can get out of sync with you actual average MPG—especially if you recently edited the mileage on several entries. I just updated your account, so the average is in sync now.