One possible reason is that the data was exported as text instead of numbers. If Excel sees 1234 as text, it’s the same as trying to ask Excel to sum “AB” and “XYZ”.
To convert all of the "text" to numbers, first highlight all the fields you'd like to update:
Then click on the "!" symbol to choose "Convert to Number":
Once converted then the totals should populate at the bottom as expected!
To bypass this need to convert in the future, right click on the data itself and choose "Copy Table" rather than the Export to Excel feature:
Then simply open a new Excel document and paste it in there. This "Copy Table" option should be available anywhere you see tables of data like this one and pasting as such will ensure the numbers are seen as numbers.