It may seem most fair that if you did 10% of the work between the time blocks A and B were found, then you get 10% of the coins paid out for block B. Because "you did 10% of the work needed to find block B".
That may seem like a fair system with no loophole. Unfortunately this is the proportional reward system which has a nasty loophole that gets abused. It is a very unfair reward system. Pool hoppers get paid much more than the 24/7 miners, on their expense.
PPLNS ("pay per last N shares/shifts"), pays the last N amount of work, for some number N. It may feel unfair if you did some work and it was no longer among the "last N shifts" when the block gets found so it didn't get paid. Then a big miner arrives, does a lot of work, after which several lucky blocks are found, so their work gets paid multiple times. It may feel like they are using a loophole. In fact it is completely random and could easily have been the other way around.
With PPLNS when you do a bit of work it is completely random whether it gets paid zero, one, two, or even more times. There is no way to predict it. There is no loophole.
If you feel bad that a big miner did most of the work in a shift that got paid 3 times because we found 3 lucky blocks, while you got "squeezed out" of this shift and didn't profit as much as you would have hoped - it was random. It could have been a shift that got paid zero times. In that case the big miner saved you by taking a larger share of this unlucky period of mining.
Mining is a lottery. Sometimes you get lucky and other times unlucky. The important thing for fairness is that the reward system makes it impossible to know whether future payouts will be high or low. That ensures a fair system with no loopholes.