I think what’s going on is that protection legendary is mainly a damage-increasing legendary. If you change your strategy to move the toughness slider to DPS, it picks it. When you move it towards toughness, the value of DPS is reduced enough that it’s going for the extra versatility socket.
In this particular case, you could just lock in the legendary if that’s the one you want to use with a more toughness-oriented strategy. That would be a better way to customize the result.
Yeah, you don’t have any other defense-oriented legendary powers, so it is choosing it purely for the stats. You have several haste/mastery items, but that one has the socket which pushes it over the edge.
The optimizer is pretty good at finding these “non-obvious” solutions that most people wouldn’t think to try out.