It's called proration. Standard practive for all phone companies. Based on when you started and what your billing cycle is your first bill is prorated meaning you don't get the entire 500 minutes since you are not paying for the entire month.
This doesn't entirely make sense then*Chris* if a person is paying for a certain plan you'd expect to get that service (plan) no matter if you started mid month or on right the day of billing cycle. Only difference should be the price due to when you first got the account established not the plan. (Sorry writing this on my cell, harder than I thought lol)