Feed.php generates a unique ID every time a user loads the Recent Changes feed.
According to lines 256-257 of that file, this is because "Atom 1.0 requires a
unique, opaque IRI as a unique indentifier [sic] for every feed we create." But
"feeds" in Atom aren't the documents generated in response to a user request,
they are "lists of related information," and an "Atom Feed Document" is merely
"a representation of an Atom feed," it isn't the feed itself.
Moreover, the "atom:id" element description states that "When an Atom Document
is relocated, migrated, syndicated, republished, exported or imported, the
content of its atom:id element MUST NOT change," and IDs on blogger.com, whose
programmers had a major role in the development of the Atom spec, don't change
when a user reloads them or even when a new entry gets published to them.
Feed IDs should remain the same between feed loads in MediaWiki too.