SH51s disengage with twisting outwards or inwards - if nothing gets in the way and you have the range of movement. However they're primarily designed for an outward twist of the heel.
It's quite common when twisting inwards to hit the crank or chainstay; hitting a chainring guard is also quiet possible if one is fitted. So that could explain why you have to twist your right foot out.
If your left foot must twist inwards, and assuming there's no reason why your personal range of movement isn't limited, I'd look at the cleat position:
- is it the same (mirror image) on both shoes
- could some aspect of the sole be hitting part of the pedal? I've had to file down the cage on my M424s to suit my shoes (which are selected for being good to walk in for commuting but also light hiking on bike tours).
I'd also consider where your foot is in the stroke when you unclip. I normally unclip with my foot at the bottom (between the 5 o'clock and 7 o'clock positions, by twisting out), occasionally at the top, but at the top I twist my heel in. I can't unclip with my right foot back (9 o'clock) because I'd twist inwards and my lock is attached there in the way. Neither could I unclip by twisting inwards with either foot forwards because I'd kick the crank.
Looking at your photo, the cleat angle and fore/aft position are almost exactly the same between the shoes:
The blue lines are drawn from the centre of the outer cleat bolt, through the centre of the inner bolt. On the right shoes (left in the pic) the line intersects a small groove in the tread; on the left shoes (right in the pic) it's slightly further forwards. The gap between the unused bolt hole and the back of the cleat recess is 5 pixels different in 90 pixels, so nothing basically. Side to side, the left shoe has its cleat 12 pixels closer to the crank. That's about 1mm, which might be just enough to make a difference.
I have had some success working out where shoes and pedals clash by rubbing chalk on one and looking at where it rubs off on the other. Some moisture may help.