[kem]

[ QUOTE ]
mBpoker

an ace should be dealt on the river 1/13th of the time b/c on average an equal ammount of each card will have been dealt

[/ QUOTE ]

That is only true if the river is dealt on every single hand, which it isn't.

Imagine a hand where three players hold a single ace each, and there's an ace on the flop. The probability of an ace on the river is zero. Contrast to a hand where a single player has an ace, and there's an ace on the flop. The remaining players fold, and no river is seen. It's not hard to imagine that the probability of seeing an ace on the river is actually less than 1/13.