I believe this is because one of the users improved the edit instead of just approving it.
There is an MSO question about this. I think this is what triggered the screenshot you posted:
When the reviewer chooses to improve the suggested edit, they also choose whether or not to approve or reject this edit. The edit will be approved by default, but the reviewer can override this approval with the check box titled "Suggested edit was helpful" at the bottom of the edit window
If the reviewer chose to mark your edit as helpful, Community ♦ will then approve your edit and you will gain two reputation as usual. Otherwise, Community ♦ will reject your edit and you will not gain the two rep
A user with full edit privileges saves an edit approximately at the same time
If a user with full editing privileges begins editing the post at the same time , then edit will be overridden in favor of theirs, if both are saved at approximately the same time.
This is known in software as an optimistic lock. We can assume that this is an edge case, and does not happen very often. Thus, when one person starts editing a post, we do not need to lock everyone else out. While the UI attempts to avoid these situations (by prompting you with "an edit has been made to this post; click to load"), they do sometimes occur.
This sometimes appears strange to a viewer—as if Community ♦ has immediately rejected their edit, without warning, with no hesitation.