Mining is a probabilistic process. One day you may be lucky to find more blocks than expected, and one day you may not. On average, the amount of found blocks will be near to expected.
The "Luck" term used in our pool can be described very easily. If luck is higher than 100%, it means that pool found a block in a shorter period than it was expected. If luck equals 100%, it means that pool found a block just in time when it was expected. Luck factor less than 100% means that the pool was unlucky and spent more time on finding a block than it was expected. Luck becomes inverted once Pro Mode is enabled.