Yes, the OBC isn't the most accurate thing. It does have an adjustment of sorts, but how it calculates it's milage is the position of the throttle and how much fuel would have been consumed by that throttle position, and also factors in speed and RPM.
Depending on what version of OBC you have, you can make the adjustments accordingly.
http://home.iae.nl/users/bts/obc.htm
The one you're looking for is OBC Hidden Code 20.