AlphaBeta issue

I there! I have an assingment on my college that is to write an
evaluation function based on the current state of the game and pass a
number value to the alphabeta function. The game is the "Pawn game" in
an 8x8 dimension board.

As an example our teacher game us two example functions.

One in mr_random, that only returns, in any state, a random value.

The other is mr_random_plus, that in case of victory send 100, defeat
-100, tie 0 and on other states a random number.

In fact the mr_random_plus is smarter that mr_random! I've tested by
playing against them.

Our assingment is to write an evaluation function, that based on the
current state of the game, sends a value to the alphabeta algorithm.

My problem is, i've tried to send always the same number, like "5", but
i still cannot predict what will the computer move be. He always has a
diferent move, even if i put him starting the game (With an equal state
every beginning of the game, it should play the same move).

Can somebody explain to me how the number sent influences the alphabeta
decision?

Thanks in advance.

.