I the case of a 65% (or 75%) there is only 3u between a 6.25u space bar and the arrow keys. So you can either have two 1.25u (normal sized) modifiers and have .5u of space left over, have 3 1u modifiers, or 2 1.5u keys. If you opt just two 1.25u keys, you need a blocker to fill that empty space.

It is mostly a matter of aesthetics though cap compatibility could play a role.