Wet V1 is lower than dry, so more runway is used to accelerate between V1 and VR while minus one engine. As the rotation point is now nearer the end of the runway you will have less altitude at the end of the TODR. It's a tradeoff: reduce the probability of going off the end in the stop case, in exchange for a slightly higher chance of hitting an obstacle in the go case. The choice of 15ft vs 35ft was a compromise that goes back to ICAO Standing Committee on Performance, and then Airworthiness Committee meetings in the 1960s.
Last edited by slast; 21st April 2017 at 13:48.