What have I started? Previously I wasn't trying to say anything about the mRound, just that the multiplication and division cancel out each other.
Lets check the code again:
%xpos = mRound((%xpos * 66) / 66);
The part where it is divided by 66 is still within the call to mRound. They cancel out each other.
if(mRound(%xpos) == mRound((%xpos * 66) / 66)) {
echo("Holy stuff man, math works!");
}