The Luck of a mining pool is probabilistic in nature. Imagine that each miner is given a lottery ticket for a certain amount of hashing power they provide. For illustrative purposes imagine that you were to provide 1 TH/s of hashing power and the overall hashing power in the network was 10 TH/s then you would receive 1 of 10 total lottery tickets. The probability of you winning the lottery (finding the block reward) would be 10%. So for every 10 blocks found you should statistically find 1 of them.
Now imagine that you found 2 out of the 10 blocks, this means that you found a block earlier than you statistically should have on average. You are lucky! Now imagine you found 0 out of 10. This would make you unlucky. Over the long run statistically you should receive on average 1 out of 10 (or 10%) of blocks, but there is short term variance. For a more in depth article on calculating luck check out this article.