The excel rendering of SSRS is what is making those merged cells.
If I had three fields in the report like:

The excel file would render with 7 columns ( thru G).
- Column A is from the left edge of the report to the left edge of
Name - Column B is in line with the field
Name - Column C is from the right edge of
Nameto the left edge ofDate - Column D is in line with the field
Date - Column E is from the right edge of
Dateto the left edge ofAmount - Column F is in line with the field
Amount - Column G is from the right edge of
Amountto the left edge of of the report
If I add field Description (and it aligns perfectly with Name and Date), I still get 7 columns. With field Description spanning columns B thru D, as merged cells.

If the description field was a little too narrow, like

It would endup making 8 coulmns.
- Date would now span D thru E
- Amount would now be in G
- Description would span B thru D