There's no "share luck" on the web site
Hmmm...
As for shares - OK they're the same as blocks - random luck finding them with an expected distribution defined by the CDF.
The reason it's not as obvious with shares, as with blocks, is coz there's way more of them.
In a day you expect to find about 25920 shares, per miner, so since the sample is quite large, you tend to not notice the variance.
If you get lucky and find more shares - faster than expected - then the pool may put your work difficulty up, assuming your hash rate is higher.
This will mean you'll average less shares of greater value - which is expected to end up the same.
If you get unlucky and find less shares - slower than expected - then the pool may put your work difficulty down, assuming your hash rate is lower.
This will mean you'll average more shares of lesser value - which is expected to end up the same.