Here is one way it makes sense: Migration makes it possible to close the question as a duplicate.
Of course, I don't really see why that would be necessary when "off-topic" or "unclear" are often good alternatives.
Edit: Okay, the reason I constructed was a little far-fetched. Here's another way such an event might reasonably take place. Someone might click on the wrong option when choosing a reason for closing.
Here's a third way it might make sense. Some of the people voting to close a question might not read every sentence thoroughly, especially if a given sentence doesn't look very mathematical or looks like pointless noise. Instead, they may decide only on the basis of the mathematical content of the question that while not appropriate for MathOverflow, it may find a good home at math.stackexchange.com.