Upgrading from iPhone 2G to 3GS? Check your first bill

My first 3GS bill seemed to be a little higher than I expected so call AT&T to speak to a rep. I activated on 6/19 and it turns out there was a pro-rated charge for the 3GS data plan applied to my bill AND a full monthly charge for my original iPhone. They hadn't pro-rated my original iPhone's data plan and deducted what wasn't used it so that added an extra $25. The rep couldn't explain why it happened but went ahead and deducted it. Just FYI not sure if this is a fundamental billing issue.

 

My concern wasn't the prorating. Its the fact that double-billing occurred. How can they charge you for a full month of service for old iPhone data plan when you didn't use a full month?