Rondo and Avery developed their shots
There has been no material improvement in Rondo and Bradley's shooting over the course of their careers so far (at least where the long ball is concerned). It's a nice tall tale, though.
I thought Bradley's 3 pt FG% improved a lot last season? Am I wrong?
You're not. He is wrong.
Also wrong about Rondo, who hasn't improved his shooting as broadly but has become a legitimately good shooter from the elbow area.
Rondo shot ~45% on 2-pointers outside 10 feet in his second year, and is yet to beat this mark.
Bradley has been a .400 shooter on shots outside 16 feet for most of his career. He shot .407 from three in his second season, so I'm not sure how last year was any sort of improvement.
The "improvement" story is largely anecdotal. Sure, it may happen, but it's nowhere close to a foregone conclusion that shooting ability is bound to improve over a player's career.