When searching for new posts, I see that my voting in one of the polls counts as a 'new post'. However, while the '<blah> minutes ago' entry updates correctly, the 'by <username>' is the last user to actually post a comment in the poll instead.

Result:
But jim last posted to this topic 4 weeks ago.

Not exactly world-ending stuff but still... bug?

That post includes a poll as well. Perhaps someone may had voted in that poll. That may be the reason for the post to show up in the 'New Posts'.

I find that irritating, though.

Also, the internal crontabs to update counters may run differently, since we don't update all counter every second to save CPU cycles ....

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.