network monitor(net.if.in) exceed bandwidth

[Totop_jia]

I monitor my network traffic by key net.if.in (in linux host)
at most time,it works fine
but while it exceeds my bandwidth(15Mbps),it may rise up to 100Mbps
obviously,it isn't worked well
So,I had a test,to get eth1 TX bytes by command [ifconfig]
I finded that the incorrect network monitor corresponds to the command output
How to explain this?
Is the [Tx bytes] by command [ifconfig] incorrect?
Thanks!