Bandwidth usage reporting possibly wrong?

[kaotix]

I have been noticing on my server that practically every day there is 1GB of bandwidth usage overall. I simply cannot work out where this is coming from as checking over the various sites hosted, there is no where near that kind of usage. Each site took around 200 MB yesterday alone as there was little to no usage overall (based on adding up manually the stats from each site) But looking in the overall stats it shows a very different figure.

How can I find out where this usage is coming from, if it even exists at all? It's just kind of hard to monitor peoples usage if it doesn't appear to get reported correctly in the first place.
Thanks