If it's your first bill or you changed your plan recently when you upgraded, it's probably proration. Basically they do this:
[Your plan/30 days] * [days in between cycle date and activation] + plan for 1 month cycle. It will show up as 2 lines on your bill. Read page 3 if you want to see it for yourself. You can view your bill on Self Serve, if you've registered for it.
Page 3 will also show any overage charges.
If you don't understand your bill or want to dispute charges you can call them for free at 1-866-995-6636.