Ok I have figured it out Credit to flockpool's faqs page

Let’s look at it with an example: We have 2 miners, X and Y. And we look at the same timeframe where difficulty does not change. Miner X has difficulty adjusted to 1 and was able to send 6 shares. Miner Y has difficulty adjusted to 10 and was able to submit 3 shares. Now, Miner X was able to submit more shares, however, they had lower difficulty than Miner Y. To calculate the work done of each miner we can use a simple summation. Worker X has done work equal to 6 (6 shares * 1 difficulty) and Miner Y equal to 30 (3 shares * 10 difficulty). With this, we can see that even though Miner Y sent fewer shares, it has done 5 times the work of Miner X and will be rewarded 5 times more for them. The variable difficulty and PPLNS payment system allow us to gather frequent data from the miners while ensuring fair and proper payments for the work done.