I found a work around. Place the cursor in the right most cell of the last row and press tab. A new row with the correct number of cells will be added.
When inserting a new row in a table the inserted row includes unwanted extra cells at the right side of the added row. For example, adding a row to a 3x4 table gives a new row with 7 cells instead of 3. Has anyone else seen this and do you have a way to avoid this behavior or to remove the unwanted extra cells in the added row? Thanks!