Looking at the others on http://www.mediawiki.org/wiki/API:Query_-_Properties , this embedding seems widespread.
Odd that e.g.,
a might give <a>,
a|b might give <a b>, but
a|b|c might give <a b c d>

'title' is taken to mean the ns+title pair in this context

Well, OK, but make sure things like this are documented, as people are
making scripts built around the output.

Else people will wonder "odd, the NS is already in the title. Is
telling it to us twice with no way to turn it off intentional?"