It's interesting to note that on the London album, Rosie is from "Pittsburgh, PA" instead of "Allentown, PA". I guess it must have been determined that "Allentown" was not a reference that would play to a London theatre audience.
Subtle semantics are the reason for the confusion.
Decca Broadway is the label, and so technically it is an original cast album (original LONDON cast) released on by Decca Broadway so a Decca Broadway Original Cast Album rather than a Decca Original Broadway Cast Album.
Odd that any mention of it being a London Album is nowhere to be found, it's true of the original LPs as well.