Few guesses would be:
1. The 3 poles are terminated only on two legs. The cheap thing to when you have a couple of spares sittng around and plenty of spare spaces to waste.
2. The wrong dead front was put on from a single phase panel
3. Split bus-panel
Longshot on them all but I go with number 1. I like the "service diconnect" for the fan? at the bottom of the post.
