Atebit, you have three fuel-ups on these dates: 7/13/11, 6/5/11, and 5/16/11. So the first fuel-up was on 5/16 and the last fuel-up was on 7/13. That's a total of 57 days, or if you break that up into 30-day chunks it's 1.9 months. We're dealing with whole months so the system rounds down to 1 month. We could probably be smarter about that. Maybe rounding up to 2 makes sense here.
Once again, this is a situation where you're just starting out so using "month" as a 30-day timeframe doesn't make as much sense as it will when you're 12 months in.