Every miner has a different adress, but the hash of the last block is the same for everybody.
Lets take these two adresses 9T5x6h164qpqk3s3Lunq7uAVLapQNCF7wj and 9ECPZJBtbMHyfGikdUzQuf9Z8t6gGbWh5s.
If we compute hash(previous_block_hash + Grain_adress) with each of them, we get a different result (hash) for every single adress.
If we define a requirement for the proof of luck like: result hash must contain seven "9" or result hash must contain five "9" in a row ("99999"), then only a few adresses can generate a hash with a valid proof of luck.
Lets say 9T5x6h164qpqk3s3Lunq7uAVLapQNCF7wj would have a valid proof of luck hash from hash(previous_block_hash + Grain_adress) for the upper requirement and 9ECPZJBtbMHyfGikdUzQuf9Z8t6gGbWh5s not, only the miner with the adress 9T5x6h164qpqk3s3Lunq7uAVLapQNCF7wj woulb be able to create a superblock with a x64 amount of coins transaction, the miner with the adress 9ECPZJBtbMHyfGikdUzQuf9Z8t6gGbWh5s could only create a transaction with x1 amount for this round. Other clients could verify this.
This means, that each miner can generate a normal block with any adress, but only a few miner with a lucky adress can generate a superblock for a round.
The hash of the last block differs with each round (block), because the last block is differnt, so that in the next round a different adress may be lucky.

We can check the coinbase transaction to verify the miner adress.
And you cant just easiely generate a adress that is lucky (valid proof of luck) for the current round, because:
- You first have to generate a private key in order to generate a public key (grain adress)
- You cant generate a lucky public key first (you could, the you dont have the private key to acces this adress)
- You cant know if proof of luck is valid for this adress for this round, before you hash it with hash(previous_block_hash + Grain_adress) and test the result.