Ahhh looks like you're using transact, I myself when I was with them had the very same confusion. I would like to direct you to the problem I had with them, and their MRTG.http://www.webhostingtalk.com/showth...burton+hosting
That's just a small problem we had. What caused a large amount of confusion was the fact that the usage never resets to 0 at any time. When you reach your 30 days, after that it will knock off the the 31st day that was in the stats and so on. So technically you can't really tell how much you're doing so far in the month because you always have stats in there from your last billing period. Now you're probably saying, "well that's stupid", welcome to the club! Now if you want to monitor how much you're doing, you should install your own MRTG and use traffic.php to calculate how much you've been doing and that can at least give you an idea where you stand.
My recommendation to you, would be to monitor your traffic closely, and use their mrtg and record the dates and times of when you get the values. Because when it comes time, they will say you "went over", when in actual fact you didn't. That is my warning to you.
Also if you read my post, you will also find out that the MRTG they provide means squat when it comes to billing. They don't give you access to the real usage that they use to bill you for, which is unprofessional in my opinion. For example, imagine the water company putting a meter outside of your house to show your usage, but that meter basically means nothing and they say at the end of the month you actually did this much, we don't use that meter.
So you're probably saying, "Well what's the point of the MRTG they provide then?", Welcome to the club!
From my experience the MRTG they provide, is just a nice little graphic with green and blue and purple and some other color i can't remember. Other than that it's useless for you, me, and anyone else that has a server and wants to monitor their traffic usage.