If a question is not off-topic I see no reason to migrate it.
Also it is unclear what you want to happen. A mod is not going to judge if your question is a better fit for Computer Science or Math. You have to convince them with proper guidance and a single task. Mods do not think for you. They click buttons on our behalf. Unclear flags should be declined. The flag outcome is correct, Martijn picked/typed the wrong reason.
It is your responsibility to select the correct site. I see nothing in the question that would make me think Math.se is a good migration target. But I'm not a regular there so I don't know exactly what goes and don't goes on those sites.
If you upvoted the current answer, you could try to undo your vote (might need an edit to unlock votes, uugghhh) and then delete the question yourself. You can then re-post on a better fitting SE site.
Alternatively try to rephrase your question a bit so it could go as a new question on a site you prefer. Because in general cross-posting is frowned upon. You can prevent the frowning by following the advice given by Mast:
Don't forget to add a link to your original question so people can see the original answer and don't have to rehash the same story. Also indicate in the new question what you're looking for that differs from what you already got.
The reasoning for not moving a question after 60 days is found in Disable migration for questions older than 60 days and I quote the infamous Shog9 here:
Migration works best as a way to salvage good but off-topic questions and give them new life on a site where they can thrive. But shuffling old, answered, on-topic questions around for the hell of it is a waste of everyone's time.