Now I don't understand why pools have score based or pplns systems if they don't change anything, ...
They do change "something". They make it impossible to pool hop. Yes they still have variance (just like Prop pool) but that variance can no longer be "gamed". Payouts are based on future events and don't change based of prior events. Since hoppers can't predict the future they can't predict when to hop in or out.
Yes people can randomly leave and they may get paid more or less depending on when they leave BUT they can't do it with any predictability. nor do I understand how pool hopping could be profitable. You don't need to respond, this is something I should research myself if I truly want to understand.
A simple thought exercise:
1) Imagine you have the wining hash. The next hash you generate will solve a block any block. You have two options apply that hash to a prop pool w/ 1000 hashes or apply that hash to a prop pool w/ 1 million hashes. Which do you pick? Obviously one pays out 1000x more than the other so there is no reason to pick the larger pool.
2) Now you may say "but we don't know if the next hash is a winner or not". While that is true the chance of each hash being a winner is the same. Thus the same logic applies. If you apply a share to a prop pool of 1000 shares you have a greater expected payout then if you apply a share to one with 1 mil shares. Same amount of work = 1 share is worth more or less depending on how many shares are in the round. Your EXPECTED VALUE is larger on shorter rounds than longer rounds. Now if there was only 1 pool in the world this would be of only academic value as you would participate in an equal number of long and short rounds.
3) Given that there are 2 or more pools if you can compute expected value of each share then you can simply put shares into the pool with highest expected value. Right?
That is pool hopping.
If you want it more abstracted:
Image a casino had a game where they put up $50 USD. Players get wage $1 USD to roll 3 dice and if they are all sixes the pot of $50 USD is awareded. However to make it interesting the $50 USD is split among all players with a token for the round. If you roll 1 or 2 or 3 sixes you get a token. When you or anyone else rolls 3 sixes then pot is split among all tokens.
Given the choice would you rather play in a round with 0 issues tokens, 10 issued tokens, or 100,000 issued tokens? Why? How does that related to pool hopping?