To quote the Git Book chapter “Viewing the Commit History”:
You may be wondering what the difference is between author and committer. The author is the person who originally wrote the work, whereas the committer is the person who last applied the work. So, if you send in a patch to a project and one of the core members applies the patch, both of you get credit — you as the author, and the core member as the committer.
A common reason for that when working with Github are rebase merges, where the committer is the person who did the merge. But depending on your git workflow there may be others.