[Moo] question for getTransition method

[Neurostep]

Hello all! I have a question: I looked at the source mootools and I do
not understand why the function getTransition for Fx-class looks like
that?

getTransition: function(){
        return function(p){
                return -(Math.cos(Math.PI * p) - 1) / 2;
        };
}

Why transition not simply equals (time - this.time) /
this.options.duration ?