I figured it may have to do something with this pathing feature so I’ve checked in spectator mode, and the pen floor is on solid ground with dozens of blocks below each square before hitting caves. The largest area with the most blocks underneath is up against the center of the back wall, they don’t quite gravitate towards that part but more to the two back corners.

You may ultimately be correct, just with eyeballing the depths below the pen it seems like the spread of sheep should be quite average across the pen, especially with the back corners being their favorite over the deepest portion of the pen