From the documentation:
https://git-scm.com/docs/git-pull
git pull [] [ […]]
git-pull - Fetch from and integrate with another repository or a local
branch
...
Default values for and are read from the
"remote" and "merge" configuration for the current branch as set by
git-branch[1] --track.
So
If your branch is set to "master", then git pull and git pull origin master will do the same thing.
If your branch is set to "master", Git pull and git pull origin some-other-branch will be different
If your branch is set to "some-other-branch", then Git pull and git pull origin master will be different