Musing is the front end application and it has also a back end which is recorded on the blockchain. The steem blockchain distinguishes a post from that of a comment. When you reply to a question and do not select "post to blog" then by default it is posted as comment. So it is already recorded on the steem blockchain as a comment. So if you edit that again you can edit the comment & the text written in that comment but how can you edit a comment to post it as a blog. Is it possible to edit a comment to post it as a blog in steemit either ? No. So the theory is as simple as that.
If you are really concerned about your answer to be posted as a blog then you have the option before posting that and after posting that it can not be changed because by that time steem blockchain has categorized that as a comment & already recorded and a comment category can not be reposted as blog category as this blockchain is not designed like that. That is why you get the chance to post an answer in musing as a steemit post only once.