GPRS Data Usage differences between Nokia and Sony

From my examples I can clearly see that a Nokia phone sends more information in the header, compared to the Sony Ericsson. But when checking the data counters on the phone, viewing 25 exact same pages I have designed, the phones reported the following :

Nokia 7250i:
Sent Data : 6,500 B
Received Data : 55,000 B

Sony Ericsson K750i:
Sent Data : 22,000 B
Received Data : 65,000 B

Now this is a bit alarming, as I am trying to calculate GPRS cost of usage, yet 2 phones are telling me different things.

My Calculations:
The Sony seems about right at 25 pages * (approx 777 B header) = 19,425B
Yet the Nokia should be showing 25 pages * (approx 1,641 B header) = 41,025 B

So anybody know anything about GPRS costs? Time to ask the Telco we use at work....

As for the code I am writing, it checks to see if the phone is capable of viewing XHTML from its header. If that's possible I display a nice colourful XHTML page. If that isn't found, they are displayed a basic wml (wap) page. So I don't care if its Nokia, Sony... as long as the phone reports that it can view something, I display it...