In general, migration paths are limited. Moderators can migrate to any site, but the community can only vote to close a question as "off-topic" and leave a comment or a flag indicating which site it belongs on.
However, even when a question is clearly ontopic on some other site, we generally discourage migration. That's because the person who asked ...
There are several reasons this happens:
The numbers at the top are cached and therefore may always be slightly out of date
You may have skipped an item earlier. In that case you don't have access to it, but it may still show as an item in the counter.
You may have reviewed it, but it is waiting to be completed (perhaps it needs a second vote)
There's a ...
The migrated question on the originating site is deleted after 30 days. The stub kept for two reasons:
Migrations can be reversed if the question is closed on the destination site.
It can be confusing to see a question migration when the original is missing. Remember that people on the destination site often won't be able to see deleted questions on the ...