MRTG Problem

Dear All

I have an E1 link (2Mbps) for the internet usage in my office. I have configured MRTG on Linux Proxy Machine and Router's serial interface. The problem that i have is some times in MRTG web page Bits per second increase up to 2.4 M or 4.8 M but it should not go over 2048 Kbps. How can this problem b resolved. Any assistance will b appriciated

Regards
Iftikhar Ahmad