How accurate are Slingshot's data counters?

For the past couple of months I've been running out of data in three weeks leaving me at 64kbps for the last week

I've been blaming my two boys who play a lot of minecraft and use skype (audio only) a bit - however I noticed that both of those still ran fine at 64kbps speed so they can't be sucking up too much bandwidth.

Anyway, my month rolled over on Saturday and I've been keeping an eye on my data usage. On day 1 it jumped to 2GB suddenly - I'm assuming this is the data I used in the last week when I was over my limit (Slingshot seems to transfer this to the following month)

I've installed netlimiter on all the computers to keep track of things.

It's day 4 now and I was at 12GB usage - however according to netlimiter, total data usage was around 4gb across the three computers (theirs plus mine) during this period.

The 12GB was at around 7pm - now it's 9:30pm and I've checked my data again and it's showing 15GB usage. This doesn't make sense as the boys computers have been off and I've barely used my computer (netlimiter shows 80MB used today on my PC)

The boys were using their ipod touchs and my wife was using her iphone but I doubt they would suck up 3GB of data in two hours.

My wireless network is secure (WPA) and I've checked the DHCP Client list to se if there's any unknown devices connected (there doesn't seem to be - I suppose)

My next step is to run malware and antivirus programs on all the PC's but I would have thought netlimiter would have picked up the traffic

Any other suggestions on what I can do to track down what the problem is?