The distribution of 1/Pt vs Phi seems mostly uniform, though there are some small non-uniformities. For instance, for West negative muons the 1/Pt seems to reach smaller values for North than South--but not by much.
It is pretty clear that the code isn't taking the projection into account when giving the muon track phi. This must be phi at the vertex. Unfortunately the projected position isn't filled (1E6).
Modified 04-August-2008 at 11:15
http://hep.physics.wisc.edu/~jnb/imu/04Aug2008