If your numeric display format rounds off numbers, Excel will still retain the full-resolution number in the cells. When Excel evaluates formulas, it uses these full-resolution numbers stored in the cells, not the rounded numbers that show in the display, which can give you results different than you expect.
I was able to get a result very similar to the one in your image, and I believe that with experimentation it would be possible to duplicate your result exactly. I set the format of cells F11 and G11 to a number format with no decimal places. Then I set F11 to 37.49999 and G11 to 57.49999. All I can see in F11 and G11 is 37 and 57 due to the formatting in place.
To find out whether something like this is affecting your results, simply place the cursor in each of F11, G11, and R11 to see the actual value. You can also see the actual numeric values by setting the formatting on the cells to "General", which will work even if the values are generated by a formula.