I got timed on radar twice, when I was in my early twenties. Those guns back then were built with a bias for a slow reading. On both occasions I was at about 70. I was a beanpole then, I probably threw harder for a time in my late 20’s. 70 to 80 is typical high school velocity.
Mo’ne Davis throws 70 at 13 years old.
Now you will see kids in Little League throw 70, but invariably they are husky boys. Davis is little, and a girl.
So yes, it’s the mechanics. Her mechanics are superb, damndest thing a baseball person could ever expect to see. She looks like a professional pitcher out there.
Other absurdity is, she throws a gen-u-wine 12-6 curveball. I misspent much of my youth trying to throw a 12-6 curveball. I was OK at it, moreso than many. It requires a complex bit of deftness. The palm of your hand has to be on the top front of the ball when you are letting go. Fact of the matter is, almost all 12-13 year olds are basically play acting when they throw a curveball. They aren’t strong enough to do anything but make the ball side-spin a bit. It doesn’t actually break for them until they get bigger, 14, 15, 16 years old. Davis’ curveball appears to actually break.