I can understand it, but I feel like they didn't think it through far enough. I imagine most Younglings (*shudder*) hope to become Knights. Kids want excitement and adventure until they grow up a bit and begin to understand duty and responsibility. The EU started to paint a bit of a picture of Jedi who ended up in different areas of focus when they had abilities better suited -- or less suited to Knighthood. They had Bariss Offee transfer from training under Luminara to the Healers (before Clone Wars overwrote that). Jocasta Nu's proficiency is in archivism. Given the random **** I just know, I can see that being a thing. More recently there's been stuff about the Jedi focused in botany who tend the great Tree at the heart of the temple. The Jedi who left/were exiled thousands of years before had a proficiency with alchemy that they passed on to the Sith they intermarried with...
Oh, hell. I just realized. Knights who are out there being heroic, Archivists focusing on knowledge, Healers and Botanists taking care of living things, and Alchemists messing with the natural order... All they need is a Sorting Hat.
Anyway, yeah. I feel it's sloppy writing to not figure the Jedi would have, thousands of years ago, figured out how to, ah... sort people into the disciplines best suited for their abilities and personalities. Someone like Etain would have never gotten that far toward Knight-dom. The Corellians-are-bad-at-telekinesis thing is just stupid, though. Family traits, I can see (or could, before George contradicted himself). But the notion that an entire planet's population is so, well, inbred that they all share that? Mmm, nope.