Now that I think of it, I've seen what looked like centered square rule before but it was actually counter-heweing to bring the "reference" face to within spec of the snapped line at the joint (basically correcting out of square hewing or wind but only locally at the joints). That can look like a reduction on both sides sometimes.

I don't know if that's what you have in that frame -- I can't see the details well enough in the photos to have a guess.