How accurate are the block timestamps in Ethereum?

In Etherscan/protocol-level, the block times differ by 12s. In reality, how accurate are these timestamps to the actual UNIX timestamp of, say, when the validator proposes the block?

I am running a Geth node and seeing block times that are between 10-14 though sometimes 8-16s. I can't tell if this is because my node's variance is high or the actual block time variance is high.

(Yes, I am aware that there is no "actual block time" but the variance is very high so hence the question).