In a multibranch pipeline project there is the option to build PR merged with a base branch (typically master or integration branches) to ensure that changes in the final status (merged) are valid.

In this cases the revision given by the build data refers to the merge commit, not the last PR commit in the remote

PullRequestSCMRevision contains info about the pull hash and the base hash that is internally used to notify GitHub. This info could be very interesting to third parties like pipeline steps, but the class visibility is default so is not possible to access it.