This grid layout gives you the option of determining the row count, which is set by nmaster + 1. So except for giving you a customizable grid, you also get the ability to show everything in one row, or in one column (row = 1 and row = client count, respectively). When calculating the cell dimensions utilization trackers are used to make sure all pixels are utilized. The effect is that no overlays or no gaps are present, but on the other side all cells are not always of equal size.

Example: splitting 2560 pixels into 6 cells gives you 2 cells with a width of 426 pixels and 4 cells with a width of 427 pixels. No gaps, but not equal size, an off trade I believe many would be comfortable with.

I personally want the presence of only 2 clients to always result in a vertical split. If you don't like this feature set the FORCE_VSPLIT to 0 in config.h.