I just ran into this quite silly problem. Well it seams trivial if you know how it was caused. I was copying pasting thousands of numbers from one Excel sheet to another, when I realized that the Excel cells do not add up correctly. In other words the sum was simply wrong.

So with a very much reduced number of cells this is how it looks:

Sum not correct

The sum for items #1 to #6 should actually be 195 instead of 175. Here it seems obvious because the little triangle in one of the cells already points to an irregularity, but that is not so easily detected with 75% zoom and a spreadsheet full of information and these lines did’t help either.

Sum not correct-triangle

a) Click on it and select “Convert to Number“.
Alternatively you can of course highlight the cell and
b) change it from text to number under Home -> Number, or
c) press “CTRL + SHIFT + ! ” to format the cell to number format with commas

Now it is correct:

Excel sum correct

So the best is simply to highlight all cells in the column or row and make sure they are in the desired number format.