Question about Promos

I am getting about 20 LP per win and lose roughly 18-22 per loss lol. So why is it then that when I lose a promo I drop back to 65 LP but when I win a Promo I don't start in the next division at 40? During the Promo I am at 100 LP which doesn't change so why are only my losses counted during the promo?

I understand if you can't just stay at 100 LP after losing a promo. However it feels like getting kicked off the top of a mountain when it happens.

Not trying to complain here, just was wondering if anyone else has felt the same or had some thoughts.