Personal rank math

Hi,
I’m a WoW player and started to play HoTS with my son. I introduced him to HoTS and we played for a season on my account. Then we started a new account and I played on my account where he played on the new account.

We did the placement matches together and we won 9/10 games. I was placed bronze 5 and he was placed bronze 2 (which already does not make sense).

Every game since I’ve had a negative personal rank adjustment that now at silver 5 has reached -24 points, while he has 0.

Our distance is ever increasing which definately does not make sense. We are winning/losing the same games and our distance is increasing while it either should remain the same or decrease so that we finally are at the same rank.

How does it make sense that two people winning/losing the same games have their ranks diverge?

Finally how does it make sense that for every 1 win & 1 loss I lose 50 points???

Old account - that means the sistem has more history to compare games and think you dont belong in actual league, try to drag you back to your league. Less points to victory, more points taken on loss. The higher you go, the harder it gets, we are all in this situation, more or less.
And your son account, since its a new one, he start with a higher MMR than you.