Probably so one can sit in the seat crossways, or at an angle. Kind of hard to explain, but I can see the idea behind it.
As far as leveling it, if it is going on a level concrete surface, probably not a problem, if it is, shims can take care of it. If it is going on a gravel or grass surface, no problem.
The Left-hand, Right-hand set up is my only question. Wouldn't it be easier just to make it either Right or Left handed, rather than ambidextrous?
By the way, very nice job, I'm jealous, and am thinking how do to one myself. I just use about anything available including car and trunk hoods, old shop built picnic tables and a wooden wire spool that the Electric company left behind.