Here is what is likely happening.
A Git commit will contain the name, email address and timestamp of two actors — the author, and the committer.
As per the Git - Viewing the Commit History info
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.
The commit view shows when the content was committed, using the committer date.
The blame view shows when the content was authored, using the author date.
You can see both of these dates in full by providing appropriate options to Git log.
$ git log -1 --pretty=fuller ce40c0f8f02e83892eb185aabea306ee2a3ab10e
Author: genjix <email@example.com>
AuthorDate: Sat Oct 29 11:39:58 2011 +0100
Commit: genjix <firstname.lastname@example.org>
CommitDate: Sat Oct 29 11:53:18 2011 +0100
BIP 0001, 0011, 0012, 0013.