The Y fare does make a difference, actually. It can vary from airline to airline and even with sub-buckets of Y fares but in many cases, True Y fares allow access to preferred seating, which as you found out, when booking on a 767-400 operating a domestic flight, means access to what is really "Comfort Plus" seats within the Main Cabin since the cabin marketed as "Comfort Plus" is the cabin with the Premium Select seats.

AA and UA do the same thing with their respective equivalent of Premium Economy cabins on many domestic routes where there is no Premium Economy/Premium Plus fare being sold. The PE cabin on AA is marketed as "Main Cabin Extra" and the PP cabin on UA is marketed as "Economy Plus." (To make it even more confusing, AA and UA do have some domestic routes where they sell Premium Economy/Premium Plus - DL does not yet).