1 pixel variation in screen positions

[Nodlehs]

I am using this function to determine the absolute vertical location of
an element in my pages.

function getYPos(element)
{
var yPos = 0;
var elementIterator = element;

while (elementIterator)
{
yPos += elementIterator.offsetTop
elementIterator = elementIterator.offsetParent;
}
return yPos;
}

Now, when I call this, in FF(1.5.0.6) I get a pixel above the element I
pass.
In another machine, FF(1.5.0.1) I get the top most pixel of the element
I pass.
In IE 6 I get the top most pixel of the element I pass.

Can anyone explain to me why I possibly get different values on
different browsers? I have tried it on multiple machines with
FF(1.5.0.6) and get the same result. Any help would be much
appreciated.

Thanks,
Ken.