I can remember this problem with older Excel, even on Windows.
The problem is that Excel thinks these columns are part of the workbook. I am sure there was a way to select just what you wanted and tell Excel the bounds, but I cannot remember.
If you go into page mode you can see what Excel thinks is your data. I think you can fix it here.
The easy solution is to select the column to the right of your data, extend the selection to the right and delete the columns.