We distinguish between "canonical" and "subjective" commits. The latter is what each validator sees locally when they decide to commit a block. The former is what is included by the proposer of the next block in the LastCommit field of the block. This is what makes it canonical and ensures everyone agrees on it, even if its different from the first +2/3 votes they saw that caused them to commit a block. Each block contains a canonical +2/3 commit for the previous block. See the canonical field in the /commit rpc endpoint which switches from false to true for block H once block H+1 has been committed :)

We should definitely document this better.

Thanks #2768 for asking about it.