They learn between model iterations. You're right, it isn't the same thing as Junior developers' competence improving with experience - the current model's weaknesses are locked in. But it does mean that much of the Junior level thinking and mistakes will be outgrown by successor models.

But they don't retain anything from your on-the-job training. The next model iteration is yet another junior fresh out of college, and knows nothing about the painful training procedures its predecessor put you through.