Sam,
I'm note sure where you got your figures for endurance and range from, but my explanation is what you need to know for the JAA ATPL POF exam. (Which probably means it is not what you need for your interview!!!)
To understand the effects of contamination on V1 we must first remember what V1 is. In effect it is the speed at which we change from being obliged to stop, to being obliged to go in the event of a single engine failure. (I know its a bit more complex than that, but you did ask for simple explanations). This means that at V1 it must be both possible to continue the take-off safely within the remaining take-off distance available or to stop within the remaining accelerate-stop distance available.
Surface contamination has two contradictory effects. Firstly the reduced friction coefficient makes it harder to stop, so V1 must be reduced. But as the depth of contamination increases, it also increases the rolling resistance of the weels being pushed through the contaminant. This tends to offset, but not entirely negate the effects of reduced friction. The overall effect of these factors is that V1 must be reduced by a significant margin when the runway is wet or lightly contaminated. But the degree of reduction required decreases as contaminant depth increases. If you still have a copy of the CAP 698 Performance Manual, you can see these effects on pages 72 and 73.
JOHN,
My comments regarding Sam getting grief were not aimed at you. I agree that a lot of valuable information and excellent explanations can be found by searching this forum. But anyone who goes to the trouble of asking a question here deserves an answer and he did not appear to be getting many.