What does UserA committed with UserB 13 days ago on github mean?

Question

I am interested in knowing which one of the two users made the file changes when github lists both. The git record contains only UserA however.

Answer 1

UserA is the one who actually made the changes. UserB is the one who committed those changes to this branch. i.e if UserA commits his changes to branch1, UserB comes, commits a couple changes to branch2, rebases branch1 with branch2. Now, topmost commits in branch1 will show that UserA has committed these changes with UserB.

Edit: This mainly happens during rebasing and cherry-picking, since authors and committers can be different in these processes.