Can anyone shed some light on the following?

My rig is reporting 4.2Mh/s average ( http://dump.chiko.nl/g/b1a725.png ) yet on the website it says 2.7 ( http://dump.chiko.nl/g/7983b5.png )

This may be paranoid, but I'm starting to think maybe my sgminer binary is skimming off my hashrate somehow.