It looks like I have found a minor bug. I am using the Developer Editor, on premise, version 2.8.0.
I have a column which is set as a formula. Its format is set as dollar. Its thousands separator is set as comma.
If exporting the table to excel, any value over one thousand is recognised as text in Excel.
This seems to happen if the thousands separator is set as comma or a space.
Changing the format to remove the thousands separator sees it correct display in Excel.