Vehicle Buoyancy

I'm working on a vehicle script that's a sort of hovercraft, but which can be raised to any height.

I've turned off hovering (hover height and efficiency are both 0, hover timescale is 1000). So that gravity won't affect it, I set VEHICLE_BUOYANCY to 1. However, when I get in, the vehicle slowly moves upward (at about 0.2 m/s). When I set VEHICLE_BUOYANCY to 0.9, I get the behavior I want.

What's going on here? Is there something else somewhere that's setting "extra' buoyancy into the vehicle? (Or could there be?) It's bothering me because the behavior I'm seeing does not match the documented behavior.

-Rob