Um, just guessing here Alex, but its not something to do with autopilot altitude hold mode is it?
As in the pressure datum which the autopilot will maintain or "lock-onto".
I know that on some ancient aircraft (eg BAe146) the altitude hold mode will allow the aircraft to climb 100-200 feet as aircraft speed increases due to idiosyncracies of the particular pitot/static system and the severe lack of a digital air data computer.
Just guessing though!