Edit: perhaps as it interacts with the definition list behaviour?

That was my guess, and the reST spec seems to confirm that, but its odd that its triggering for an enumerated list...

Optional classifiers may follow the term on the same line, each after an inline " : " (space, colon, space). Inline markup is parsed in the term line before the classifier delimiters are recognized. A delimiter will only be recognized if it appears outside of any inline markup.

   term 3 : classifier
      Definition 3.

Syntax diagram:

+----------------------------+
| term [ " : " classifier ]* |
+--+-------------------------+--+
   | definition                 |
   | (body elements)+           |
   +----------------------------+