Combine versus merge
While combine and merge are generally considered synonymous, Goodreads Librarians often use the two words to mean distinctly different things.
Combining means taking two or more editions and linking them into one work. No editions are deleted, so you have the same number of editions before and after a combine.
Merging means combining two editions that are actually duplicates, and then deleting one. An edition is deleted, so after a merge, you will have fewer editions than before. If a merge is performed correctly, all ratings, reviews, and shelvings of the deleted edition will be moved to the edition with which it was merged (or the most popular edition, if it was merged with a group of editions).
In both cases, the book page's cache may take a couple days to completely update.