Interface Status Traffic Counter Resetting?

I've noticed that the traffic counter under Status:Interfaces, next to
In/Out Packets seems to reset itself after a large (possibly around
5GB) of traffic has passed - I checked it again today, I downloaded a
large DVD .iso which took the counter to above 4GB and after using the
net for a while I've just checked it again and it now says 1.52GB. The
system has not been reset (the uptime confirms this). I had a look in
the known bugs section and didn't see this listed - does this happen
to anyone else?
Thanks in advance.