Sometimes mathematics and statistics (as well as mathematics and TCS, or mathematics and computational science) overlap.
There are times (such as for this question) when I'd like to be able to migrate a question to a more suitable site, even if it's up to debate whether it belongs to the set "research mathematics questions" or not.
Unfortunately, the mod tools do not allow me to vote for migration without first clicking on "This question does not appear to be about research level mathematics within the scope defined in the help center.".
For instance, I voted to migrate the question linked above to stats.stackexchange, but this does not show up in the closure reason, only the text "This question does not appear to be about research level mathematics". A new user may be put off by this stronger opinion (which does not reflect my view).
So, I'd suggest to put the option "This question belongs on another site" in the top-level closure menu rather than inside the submenu "This question does not appear to be about research level mathematics".
(incidentally, I have noticed that the option "This question belongs on another site" does not appear in the closure menu for every question -- why?)