The answer of François G. Dorais at https://mathoverflow.net/a/23200/12357 has no user name displayed. Is this a bug?
That answer has what we call community wiki status applied to it. This is a special sort of status, wherein anyone with at least 100 reputation can edit the post it's applied to. If you look at the revisions of the post you can see where it's applied. It's not a bug. The implementation is rather confusing at first glance, we're working on improving that, for exactly the reason you just raised.
However, it's generally a good idea to avoid wiki status unless you want exactly what it's designed to deliver, making a post editable by a much larger group of people. Since such posts can be edited by almost anyone, the original poster earns no reputation for votes received after the status was applied. Sometimes, it's used for that purpose - but it's a good idea to stay away from that. If it's good enough to post, and doesn't need to be editable by a large group of users, then you should earn points for it.
A great use is a community curated canonical question / answer that addresses a very common, frequently asked question.
More on the future of community wiki can be found on the Stack Exchange blog, but offered as advice. You should of course use it in the way it works best for you.