Currently, two sites across the Stack Exchange network are explicitly mentioned in the vote to close reasons:

Super User is mentioned explicitly in a close reason.

reason. Server Fault is mentioned explicitly in a close reason.

And there are five sites offered as migration paths from Stack Overflow:

Meta Stack Overflow

Super User

Tex

Database Administrators

Cross Validated

I understand the difference between when you should use a close vote versus a migration vote.

I understand there are a limited number of custom reasons, so not all of the migration paths can also have special reasons in the custom close reasons. I also understand why some sites aren't offered as migration paths (like Programmers).

But...

why does Super User need both a close reason and a migration reason?

why does Server Fault need a close reason (which recommends the user might have a question suitable for that site), but it doesn't need a migration path?

why do Tex, Database Admins, and Cross Validated need migration paths but do not need close vote reasons?

I'm interested both in specific reasons for these specific sites being set up as they are (migration path versus close reason and why some have one, some have the other, and one has both options).

Are we implying that SO close voters can't be trusted to migrate good questions to Server Fault, but they are good at identifying questions that are off-topic on SO and might be fit for Server Fault?

Are we implying that SO close voters are good at identifying questions that should be migrated to, say, Cross Validated, but we don't see questions asked that might be fit for Cross Validated, but we should just close them and point the user to the Cross Validated help center?

Are we saying that we see both questions that would be a perfect fit for Super User and can be migrated as is as well as questions that might be fit for Super User but would probably be better simply closing on SO?