I agree; good question.
The FMC doesn't have to allow 3x drift because it flies constant radius turns at each end of the pattern by varying the bank angle to allow for different groundspeeds due to wind.
When you fly a raw data hold, you have no idea what the groundspeed is doing as you go around the turn, so you fly a constant bank angle. This results in turns of different radius at each end of the pattern (assuming a crosswind component). The only way to compensate for this is to exaggerate the drift effect on the outbound leg.
The FMC flies perfect constant radius turns, so the outbound leg can be flown parallel to the inbound leg.
Hope this helps
Eck