Eric, Eric, Eric... <grin>
Your ex-75B SA is, indeed, very tight. It is also very NON-STOCK. The frame and slide have obvously been polished and messed with, perhaps tightened. And its truly slick -- it feels like its on ball-bearings. The gunsmith who did the work for you was gifted, as you well know.
But, none of the other 75B SAs I've handled are as nice or as slick.
And, while its accurate, I wouldn't really say its more accurate than my 85 Combat, which has had some tenderness applied, but nothing like the 75B SA. And, the little Compact I bought from you seems to shoot right with both of them, even though the trigger isn't as nice. Its no slouch -- but the trigger makes it harder to compare them evenly. Not quite in the same class.
I suspect that some 75B SAs are very accurate, and some are only so/so. Ditto the 75B DA/SA.
I can't find or see a thing that is built into the 75B SA that should make it inherently more accurate than its 75B DA/SA brother or sister. The barrel, the bushing (non-removable), and the lockup seem the same. Slides will interchange with a non SA equivalent.
The slide design is a little different -- longer rails -- but those don't really come into effect until the slide is so far back that the bullet has left the barrel. The 9mm version doesn't even incorporate the Full Length Guide Rod of the .40 version. The frames themselves seem identical, except for the beavertail, and all the parts interchange, except that that SA has a couple fewer parts.
It would be interesting to find out if extra care is taken with these guns through production. You have contacts (or did) at CZ-USA. Why don't you ask?
You may be right, but it isn't intuitively obvious why they are more accurate, if they are.