Pool luck refers to the ratio of the actual number of block out per day to the number of theoretical block out per day in the pool. The block out of mining pool is random, so the speed of block out is uncertain. In theory, pool luck will float around 100%.
Example: A cryptocurrency pool, in theory, can mine 10 blocks per day. If there are 12 blocks out on a certain day, it's luck is 120%; if there are 9 blocks out on a certain day, the luck is 90%.
Factors affecting luck of mining pool include instability, node status of mining pool and code deployment, etc. If a mining pool's luck is low for a long time, the above factors may be considered. Pool luck only affects users who choose PPLNS, PPS+ payment methods. And it does not affect PPS, FPPS users.