[zamadatix]

You can fly over the tropopause depending how high it is at the specific location (can be lower or higher than the numbers listed) but, by definition, it'll only be "common" to observe turbulence in the range 99% of planes actually fly. I wouldn't read too much into that. As some others pointed out the height difference probably more due tot he space being faster and unused since commercial flights stay to where is more efficient.