Sometimes a bug in Excel results in the application displaying the text of a formula rather than the result of the formula in the spreadsheet. I have not been able to find a pattern of when it does it, but I have some spreadsheets that do this consistently.
To get Excel to properly display the result:
Select the cell.
Format the cell as "General". (Right-click the cell, select Format Cells, and choose "General.")
Delete the "=" at the beginning of your formula, and hit Enter.
Insert the "=" back in the formula at the beginning.
That is it. You should now see the result of the calculation in the cell rather than the text of the formula.