[foo101]

This is true only if the merge operation leads to a new commit or altering of existing commits. But your parent comment was asking this question for merge operation where there is no new commit.

The person who merged the commit appears as the committer name only if the rebase led to a change in the SHA1 hash of the commit.

If the pull request branch is already based on master and up-to-date with master, then "git checkout master; git merge pr" performs a fast-forward merge without altering any commits. The SHA1 hash of the commits in the pull request do not change. The merger's name does not appear anywhere because the merger has not done anything except fast-forwarding master.