[Bootvis]

This can have another explanation as well: the moment a block is found, the miner starts building on top of the previous block but hasn't constructed a new full block of transactions yet as that costs a bit of time to calculate and distribute. In this period, a new block could be found.

[thesz]

Blocks are Merkle trees, only the head transaction contains global seed. So, for one to mine block, one needs to walk Merkle tree up from head and then finish work with small amount of data in the block header.

Thus, the time spent mining block is directly dependent on the logarithm of number of transactions in the block.

If one can mine a block with 3000 transactions (11-12 hashes to the header) in 10 minutes, one can mine a block with one transaction (1 hash to header) about ten times as fast.

The construction of the block is negligible if we talk about complete block mining time.

[Bootvis]

This is incorrect. The miner just changes the header of the block and rehashes. The transaction set is fixed for many tries.

[monerozcash]

>If one can mine a block with 3000 transactions (11-12 hashes to the header) in 10 minutes, one can mine a block with one transaction (1 hash to header) about ten times as fast.

Huh? Surely the attempts for both take exactly the same amount of time after you've initially constructed the block, you're calculating only a single hash for each attempt.