Checks / statuses for merge commits aren't actually for merge commits

This piece of UI strongly suggests that the checks are for the merge commit:

However, they are not:

Merge commits actually show checks for the last commit in the branch, which is a different state of the codebase than the merge commit.

This feels strange / wrong / confusing; any idea why it was designed this way? Is this behavior documented somewhere? (Couldn’t find it.)