I've collected PRR ETT's for just about fifty years, and I've never encountered any explanation of the rationale on that one.
Just to cite one very confusing instance of Pennsy nomenclature, the Northern Division's Emporium-Erie line was long designated as the "Main Line-Erie", presumably because it represented the former main of the old Philadelphia and Erie, which usually saw less traffic than the Emporium-Buffalo line later designated as the Northern (or Buffalo pre-1956) Division Main.
The Main Line-Erie saw passenger service until sometime in the early 1960's. Yet by that time lake ore traffic was gone, it hosted only one through freight in each direction and much of it was dispatched with the use of unattended Block-Limit Stations. PRR rules and policies appear to be full of contradictions.
There is a Yahoo group dedicated to PRR signalling and a post there might turn up some more relevant info.