I think it has to do with a few things, based on where you are stationed, the changes in atmospheric pressure with various weather and temperatures can cause the air/fuel mix to go from lean & rich, but that shouldn't vary your mileage over 5! Unless of course, you have a much older car - but since you have a computer I'm assming that's not te case.

(Newer cars sense the change and inject more or less fuel to counter atmospheric effects.)
I did notice a difference when I was stationed in Japan - I had a 1985 Integra, and I could never fill up on the set days, even if I didn't go anywhere but work. It always seemedto run lean even though I had it checked out professionally...

I also had troubles with a station stateside. I went to a Town & Country in TX nd filled up. My truck had a fuel valve problem thvery next day. 3 days later I filled up my VW & the whole next day I had nothing but problems with the fuel injection!! I never went back.
I also confronted the AAFES manager in JP quite a few times. We were paying $1.38 for almost 2 years while the states were paying $0.81 per gallon!!! She claimed it was because they bought their gas at the beginning of the year and couldn't change the price until the next year. I countered with the fact that gas had been under a dollar stateside from the previous August and if not a total idiot they would notice it was now May!! (Fiscal and standard year had come and gone.) Just blood suckers! (AAFES and gas companies!)