It's possible that it used to be in a book and someone removed it as a chapter from the book. Removal of a chapter would not appear in any feed, so that could be why we don't know what action triggered it. As of 4.0.15, similar actions were incorrectly being considered for new revisions. This is already fixed in the next release, on the grounds that it was a waste of time to process if there was no edit.
However, even if there was an action that should be considered for a new revision, if there was no change, it still shouldn't create a new revision. For this reason, I leave the report open.
I was able to duplicate the latest revision by adding the affected page to a random book. This will not be possible in the next release, but I can now look into why the page was considered changed.