Scoring software calculates a fixed correction based on the known surface height at takeoff that is applied throughout the flight. A correction computed at landing is used in the US at the finish if more favorable.

Right, so in the US, the "Scoring Software" has to compute, track and manage 4 altitude values:

1) GPS Altitude
2) Raw Pressure Altitude
3) Pressure Altitude corrected with takeoff roll
4) Pressure Altitude corrected with landing roll

And know when and where each one is applicable, and present those results transparently to the official scorer and contestant.