Why do course mastery goals disappear once a student reaches 90 percent?

Khan Academy marks mastered at 90% of the way. They mark it as mastered but at least according to the goals, it'd be optional for a task goal to be perfected. Perfection is at 100% perfect. If someone just wanted to get to mastery, they can do so, but on the other hand, if someone wants to spring over to perfecting the goal, which is more than what the minimum goal requires, then, lucky for them, they are also able to.

reply back if this is hard to understand, this can definitely be reworded

Khan Academy marks mastered at 90% of the way. They mark it as mastered but at least according to the goals, it'd be optional for a task goal to be perfected. Perfection is at 100% perfect. If someone is lazy and just wanted to get to mastery, they can do so, but on the other hand, if someone wants to spring over to perfecting the goal, which is more than what the minimum goal requires, then, lucky for them, they are also able to.

That's right.

If someone is lazy and just wanted to get to mastery…

This is precisely my point. By making the goal a "Past" rather than "Active" once it reaches 90%, Khan Academy is endorsing the very idea that mastery learning is meant to counter. They're saying that close enough (90%) is good enough. Maybe it is for some students and teachers, but for me it's awkward to explain how important it is to master foundational skills and then say that we have to go into "Past" goals in order to complete them.

Perhaps there's a good argument for effectively marking goals as "complete" at 90%, but I haven't seen it presented here.