Woof etc,
With a dry V1 you should be able to suffer a critical engine failure, right up to V1, and still be able to abort the T/O and remain on the R/W. Now, if the R/W is wet your braking action would be degraded. Thus, if you suffer an engine failure at V1, on a wet R/W, you may go off the end off the R/W before coming to a complete halt. Therefore, to guarantee that you can abort, and stop before the end of the R/W, you must reduce V1 - ie. bring your decision point nearer to the threshold at which you commence the T/O roll. This is a wet V1.
Now, for the sake of argument, let me put some random figures to this:
Dry R/W: V1=120kts and Vr=140kts
Wet R/W: V1=110kts and Vr=140kts
Note Vr is, obviously, independent of the R/W condition.
From the above figures you should be able to see that in the dry case, with a failure of the critical engine at V1, you would have to accelerate by 20kts, without an engine, before hitting Vr. However, with the wet case, you would have to accelerate by 30kts before hitting Vr. From that I hope you can see that the wet V1 case would require the ac having a longer take-off roll.
Now, the climb gradient from unstick to screen height is exactly the same in the wet and dry case; however, due to the wet case having a greater TORR, the wet case will see the ac arrive at a lower screen height than the dry case.
I've tried to avoid technical spiel with that explanation, hope I haven't caused further confusion.
S15