[gentoo-user] What is the difference between emerge's --changed-deps=y and @changed-deps?

[Morgan Wesström]

On a freshly updated system (emerge -uDN @world):

"emerge @changed-deps" wants to reinstall 0 packages.

"emerge -u --changed-deps=y" wants to reinstall 24 packages.

"emerge -uD --changed-deps=y" wants to reinstall 181 packages.

A couple of years ago there was a build breakage in Portage because, as I 
understood it at the time, some developer changed the dependencies in an 
existing ebuild without bumping its revision level. The solution was to use 
--changed-deps=y to catch these occurrences and I've been using it in my 
regular update routine since then. But as you can see in the third example 
above, it usually wants to reinstall hundreds of packages that doesn't have any 
updated versions and I'm wondering if this is working as intended. I have a 
hard time believing that gentoo devs are pushing changes to existing ebuilds in 
such numbers on a regular basis without bumping the revision level.

Some time ago I became aware that Portage now has a @changed-deps set, which I 
assumed was accomplishing the same thing, but it doesn't produce the same 
result as --changed-deps=y - usually just a dozen reinstalls or so.

Can someone please elaborate on what's going on here, what the difference is 
between --changed-deps=y and @changed-deps, if that difference is intended and 
what the recommended update procedure is these days to catch these and other 
kinds of inconsistencies in Portage?

Regards
Morgan