MC._alpha problem

[Nixy]

That is weird
I'm trying to change the MC alpha usign this code

// This is an example
var iSpeed = 5;

function fChangeAlpha() {
myMC._alpha -= iSpeed;
trace(myMC._alpha)
}

_root.onMouseDown = function() {
fChangeAlpha();
}

//--------------------------------------------------------------------

The problem is that the MC alpha does not equal 95%, 90%.... The number is not
round. Why?.

Does anybody have an idea.
I'm usign Flash MX 2004