I had a quick look and it seems that the “Select Edge” node has very specific way of presenting curves.
I drew two pairs of walls - one straight and one arched, one wall from left to right, the other opposite.
I compared their location curve and the outcome was as expected - the start points were at opposite ends.
Then I selected matching edges on each wall and the start points were at the same edge for each pair.
Once I created some simple ACs, they were placed according to the “Select Edge” curve start point - repeating your initial observation.
I’m not quite sure how an edge’s start point is defined in Revit… Maybe someone else has additional input on the subject?
It doesn’t seem to be the resulting curve’s normal, nor the wall’s project orientation.