Another angle.
I am thinking about the atmosphere being "higher" or "thicker" at the equator than it is at the poles.
Consider a flight from the north pole to the equator. If the aircraft maintains a given flight level during the cruise its actual distance above mean sea level will increase as the aircraft progesses towards the equator. FL350 at the north pole is a lot closer to mean sea level than is FL350 at the euqator.
In this sense, although the aircraft autopilot may be "locked-on" to the pressure datum equivalent to FL350 its actual distance above mean sea level will increase.
Again, Alex, just guessing.