It's all in the dates. For the $3.60 charge if you look at the dates for that usage it's 6/25-6/25 --- so the text package was only effective for one day, the end of your bill cycle, so for that 1 day you were only allowed 16 messages. So the allowance was prorated because the package was only effective 1 day. Going forward though you're given the full allowance because the package will be effective for the whole month..so you're not limited to just 16 messages per day... you have 500 messages per month.