they are set by renault using their equipment. they are correctly positioned unless you've been tinkering.
and yes, left looks brighter due to the angle it points slightly higher than the right. reason: not to dazzle oncoming traffic
Are they self levelling on the 172?
I assume so.
Mine seem to level quite high, quite often get flashed at, ohh well, not my fault they can't afford Xenons lol.