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That is actually the expected behavior, albeit not a most attractive implementation. The thread gets touched when you vote on a poll and hence gets promoted to the to pof the list in terms of most recent, but as you observed the "updated by" does not get changed because the poll vote is not a post.

The internal mechanism used by vb, for efficiency in queries, is it to take the top N list by date from the smaller table ( thread ) which contains a last post id as one of the columns and simply selects those ids from the post table. In the current implementation there is no way to tell if the last update is a poll vote or a real post.