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.
In other words, no matter how "lucky" your miner is, it all ends up the same anyway.

Well yes and no

Yes you're not expected to vary much from expected luck.
No, you can get lucky and unlucky sometimes, and find more or less shares than expected.
But a "share" itself is just a way to prove your miner is doing some work, and (in general) has no influence on the block itself, right? So the goal of the "work difficulty" set to each miner is to maintain a balance between the load on the pool (too much shares submitted needs lots of processing power on the pool side in order to handle them) and making sure the miner actually WORKS for you (and not for some other pools at the same time). Am I correct?
Well, basically yes

However, a block is a share also, that just happens to have the difficulty, or more, of a block.
So we definitely want everyone to find those shares also
