Steelbird
Sorry for the last response, the reason why the V1 adjustments are larger with the lower V1 speeds has to do with runway friction.
It is assumed that the wet braking Mu is half of the dry runway Mu. At higher weights the dry Mu starts to decrease and so the margin between the dry Mu and wet Mu gets smaller. (The wet Mu is a constant.) therefore so do the corrections!
As for the statement in the AFM dated 1-5-73.
Takeoff from precipitation covered runways
At rotation speed minus 10 kts (VR-10Kts) lift the nose wheel out of the precipitation by rotation to a three to five degree nose up attitude. Maintain this attitude until reaching the computer VR, then rotate to normal takeoff attitude.
Nose wheel and impingement drag will be greatly reduced by use of this procedure.
Mutt 