Then how do you explain setting one up with a level and a square? If it's perpendicular/plumb to the string......it's Perpendicular/plumb to the ground. Ifyou're using a bubble level to set your arrow......what's it "level" to?
The arrows elevation on the bow at rest is set in readings that are 90 degrees to the string. The string however would not be perpendicular to the ground if you were shooting at a target 20 yards out shoulder high. The arrow would also not be parellel to the ground.