I had made a edit suggestion, which was the same as this post revision. I had made the same edit before this user's edit.
Then why was mine not accepted while the same edit by this user was accepted?
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Anyway, that aside... notice the reason unanimously given by the users who rejected your edit (besides the users who approved it) — it was too minor:
This edit is too minor; suggested edits should be substantive improvements addressing multiple issues in the post.
The thing about suggested edits from less privileged users is that we want to be sure you know how to contribute substantial edits, before we can trust you to make... smaller edits. This is why the edit review system exists and why such a rejection reason exists.
Some possible reasons why your edit was rejected as too minor could be:
Admittedly, there isn't much more to address in the post (although that doesn't mean there isn't anything left), which could be why the other user submitted such a minor edit as well. But that's just my guess.
Your best course of action, going forward, is to look for other posts that have more room for improvement than just a wayward spelling error, a sign-off, and so on that you can put more effort into. Such posts are not few and far between, so you have plenty of opportunities to contribute meaningful edits to the site.