What is up with the Chess Mentor?

This doesn't bother me too much, but can someone please explain why completing a lesson with a 100% score only gives you tiny increments to your mentor rating (such as +1), yet one small error gives you ridiculous minuses (such as -19)? It makes no sense and it's a bit irritating. Is chess.com trying to tell me that 19 lessons passed is equal to 1 failed?

And I know that some people will say that your mentor rating doesn't mean anything, and its only there for show. But if it doesn't mean anything, why do you even get a rating at all?

I'm sure that new players that are trying to learn chess and care about their rating will be put off using the chess mentor due to this punishing rating system.

I think it has to do with the rating of the problem being worked on, compared to your rating.  

There is a lot of very easy stuff in chess mentor, which is good, it means you can build up confidence by getting things right. But if someone, like me, is on 1600 he needs a reality check with a few -19s on the tougher lessons.