As a sort of clue as to what is going on, notice that the discrepancy, -1.4210854715202e-014, is 2 raised to the negative 46th power. I simply means that the answer was accurate to 46 binary digits. It is sort of like how you would do a 10 divided by 3 by long division on paper and get 3.333 and then if you multiply it by three again you get 9.999. We could have carried it out to more digits, but the problem would have remained. We think in a different number base (normally) than computers.