Has anyone bothered to do the MPG calculation when filling up the tank of gas?

I noticed one day filling up the tank that that I took more gas then I expected based on the number of miles on the Trip A odometer and the MPG figure that was presented to me in the spedometer. I reset each Trip A, Trip B odometer and the MPG calculated figure each time I get gas. Lately, I've started doing the calcuation and my number is always lower then what the dashboard indicates.

MPG is simply the number of miles driven divided by the number of gallons used, yes? A simple calculation, yes? Then, where can the math error be? Does anyone know if Nissan is using an alternative calculation?

I'm probably going to contact Nissan soon for the 7500 mile service, but I'd be curious to see if anyone else out there has had a similar experience.

Thanks for your time.