Bandwidth Usage -- numbers are too low?

I know WHM's "Bandwidth Usage" routines were redone a few months ago -- kudos to the developers. Prior to that, I saw many posts about the numbers being doubled up, etc. I do not see find any recent posts on the Forum about the bandwidth calcs. If you are aware of any related posts, please point me to those threads

We have two Cpanel servers, and for both of them I find the WHM bandwidth numbers to significantly lower than the real numbers. Our numbers in Bandmin correlate very well by the numbers provided by the data centers. The WHM numbers are generally about 50% of this for each of the IP's -- it seems like the same thing is consistently left out for every IP.

My gut feel (and I cannot substantiate this) is that is has to do with email traffic. I think email with multiple recipients are counted once (instead of the message size times the number of recipients) and also that email sent via Webmail is not counted. But, as I say, these are just my impressions, not facts.

If you have a similar experience or explanation, please let me know.

PS: Is there a one-page bandwidth summary for resellers on a server? I currently use the Reseller Center | View Usage/Stats and Manage (suspend,terminate,etc) to view them one by one.