I thought the FCC required equal EIRP of both vertical AND horizontal
polarization for an FM broadcast station? Did the horizontal requirement get
deleted?

I don't know, just reporting on the results of my tinkering, I get by far the
best signal with vertical polarization. I assumed that was becuse they wanted
to target automobiles. Maybe the EIPR of the stations in both the vertical
and horizontal planes in my area are not in compliance?
73 Gary N4AST