I am experiencing the same thing, even with mysql log.

I am logging on all interfaces, and my server is a gateway to internet.

And from what I understand :

my server external IP logging is accurate

my lan client traffic is at least doubled. I think that bandwidth monitor log either on eth0 (lan) and eth1(wan) the traffic for my clients. Further more my external connection use ppp, so technically there are 3 to monitor with "any" ...

total traffic is totally outsized. ( 2 or 3 time even considering local traffic.)

indeed logging only my external