Thumb[hang]nail

I’m using WP 2.9.1 and NextGEN Gallery 1.4.3. I’m trying to get the thumbnails to display 20 per page, with 5 columns and 4 rows … but instead of displaying 5 images per row, it shows 4, and drops one all by itself to the line below, then drops another line and starts again, like so:
X X X X
X
X X X X
X
and so on. My thumbnails are 100×100. The theme column is 651px wide, which is more than enough room to accommodate 5 100px thumbs, even taking into account the padding, margins, etc. If I change the number of columns to 4, I get this result:
X X X
X
X X X
X
and so on. It seems nothing I adjust will prevent that last thumbnail of each row from dropping down.

Ideas from you WordPress gurus out there?