I had the same problem once with my MS excel worksheet before.. When I input a formula in a cell and copied it to other cells, it copies the formula but displays the one from the originating cell. I had to press F2 then hit enter to "refresh" the cell. It would now then display the correct figure for the copied formula.
I didn't get to fix this problem with the said worksheet though. I just opened a new worksheet and all worked fine. I guess there was just some sort of bug with the file itself. Hope this helps.