Broadband usage figures incorrect?

[#1]

I am a new member and have been with Plusnet less than 3 days. I was previously on an unlimited download plan with Orange.
As you can imagine, now I'm on a 60GB limit  I'm a little bit paranoid about how much I'm downloading so I have been keeping a close eye on the figures.
I know for a fact that I have downloaded around 5.5GB so far. Of that amount  4.9GB has been between 12am and 8am ( free period) and has nearly all been P2P stuff except for a small itunes update (100mb)  and open office update (125mb). That leaves around 600mb of paid for usage.
According to the online statistics my total usage is 1.22GB and the paid for usage is 580mb.  The paid for usage roughly matches but the free usage is way out at approx 4.2GB less than my actual free usage.
I'm starting to worry that the online figures are not very reliable and can't be trusted. At the moment the most important  figure which is the paid for usage appears to be correct but who's to say it will stay that way !
Is it normal to see these discrepancies between actual figures and online figures?

[#2]

You could well see less usage than actual shown in the free period,  this quote from Mand in this thread http://community.plus.net/forum/index.php/topic,86232.0.html

Quote

IIRC the script run to collect the usage stats is run at 15 minute intervals, but we generally discard the last one in the free period and the first one in the paid period and the last one in the paid period and the first one in the free period. This is to make sure you don't get charged for usage which should be free.