Okay, I'll give my reasons.
Firstly, it would be an interesting difference between the two craters. In Lix, the imploder and exploder ("fling bomber") have different-shaped craters, the former resembling a Lemmings bomber and the latter circular, and this has meant designers sometimes have a bit more flexibility when one or the other shape would be preferable. Of course, the grenader and bomber are much more different from each other, so the designer won't have that luxury of choice, but it's still nice to have these little distinctions so that skills aren't just blindly copying each other, similarly to how the miner and fencer tunnels are distinct.
Secondly, it makes sense. Lemmings explode with an irregular crater, taller than wide, because the lemming himself is irregular and taller than wide. The grenade is a small circle.