In the left_side.htm, you are arranging 3 rows with 4 images each, in the right_side.htm it looks like you are calling 2 rows of 6 images each (#gallery_6) which are trying to fit as best they can into the 4x3 layout.
I think I got that straight. In any event, that's the short answer. Does that help? Or do you want to modify the layout?