When the target user does more than one subsequent edit and the operator specifies -rollback, the edits are skipped and not rolled back.
The reason is the following call:
page = pywikibot.Page(self.site, item['title']) history = list(page.revisions(total=2))
This caches the two newest revisions.
Then, in APISite.rollbackpage the cached revisions are iterated over, as if they were all the only existing revisions (page._revisions).
If both of them were made by the same user, the method raises "Rollback of {page} aborted; only one user in revision history".