Pull request's base SHA is different than result of `git merge-base`

Here is the pull request in question: https://api.github.com/repos/getsentry/sentry/pulls/21887

Notice that base.sha = ca883c09e547d851db60d51be44ec849d667061c if I checkout the branch locally and do git merge-base HEAD master, the commit SHA I get is 7792206ddf1de9eaae9f2cbab7e51a9d06d3f9c2 which is what I would expect.

You can test this yourself since the repo is public:

git clone git@github.com:getsentry/sentry.git
git checkout -t origin/build/gha/refactor-acceptance-timeout-python-parallelization
git merge-base HEAD master

After reading @brightran’s answer to a different post: How to get base SHA when using `pull_request_target`? - I think this might be related in that the merge base sha from the github event refers to the base sha at the time of opening the PR.