I don't think my answer would be the most accurate but I guess ill just tell u what I believe.
Because of Sawamura's flexible body and how he grips the ball in random ways the ball spin changes every time he pitches. The reason this is possible is because Sawamura never had any proper coaching before he came into seido, thus he never pitched the typical "proper" way before he came to seido, unlike other pitchers who learned the typical form of pitching. That's why Sawamura's pitching is so unique and no one else is like him. And since his fastballs moved because of the random grips it would be hard to control them, which is why in act II he and Miyuki make a thing called numbers. I'm pretty sure in reality it would be hard to be this good, and ofc I believe that Sawamura is somewhat special. and the reason he doesn't pitch his random fastball is because after Chris taught him the 4 seam fastball his balls start to move lesser, so instead of using his moving fastballs, he uses his 4seam that is hard to time and has great location.
The way he learned the cutter so fast was because he learned it naturally in a game against Sensen. Because of his flexibility, he was able to throw a crossfire pitch naturally with ended up being a cutter. Now his change up. Coach Ochiai explains that to throw a change up u just have to swing ur arm thru like a fastball. And well, it ended up Sawamura was a pro at that.
Well that's what I think. This isn't the most accurate explanation but hope it helped