Publisher's "publishing history" graph shows other publishers' editions

Whereas the rationale for the graph labeled Publishing History is "This graph charts editions from this publisher over time", it appears that it also charts editions from other publishers who published other editions of the works that are in the first publisher's catalogue.

In pseudocode:
Let P1 be the publisher under study, who published edition E1 of work W1. Other editions of W1 are E2, E3 and E4, published by P2, P3 and P4 respectively. The graph then charts not only E1 (as per the description), but also E2, E3 and E4.