I think we would need to see the spreadsheet to explain it, but there is one thing that might be causing it. Excel could be treating one or more cells in the row as text (even though they are numbers).
In my example, I have values formatted as dollar currency, but Ive entered the value in D4 including the dollar sign as well, as you can see if you view this cell in the formular bar. This is treated as text, and omitted from the total in H4.
However, if you clear the contents in D4 and enter 35 in cell G4 the total increases to the correct figure of 277. I think that this is the behaviour your describing.
If you can't post your spreadsheet, try the following:
1. Try to get a verification of your total, using either a calculator, or by re-typing the numbers into another row or column (just the numbers, nothing else). If you get a different total, look for a value equal to the difference in the original row.
2. Inspect each cell value in the formula bar, looking for anything different compared with other cells (particularly the one whose value is the difference, if you can find it).