Click the "Formulas" tab and then click "AutoSum" or another function in the Function Library group.

I tried a variety of different input scenarios, including the possibility that the bottom two cells in the list were left out of the autosum formula, but I haven’t been able to reproduce ...

When you click Excel's AutoSum button (the one in the toolbar that looks like Σ) from a cell next to a row or column of numbers, Excel automatically sums those numbers.

Excel: AutoSum from the keyboard: Select a blank cell to the right of or below your values and press Ctrl+= (that’s Ctrl and the equal sign) Word: A fast way to cycle through all open documents: Press ...