When reviewing edits through the review queue, people quickly glance at an edit and have their mouse pointer already over the appropriate button (Approve, Reject). At my first glance (less than a second of looking at it) this was a user attempting to improve the formatting on a post. Only because you were seeing an issue with this edit did I look at it closer, and saw the obscenity.
The review queues make people more likely to decide on first impressions, and then edits such as these can slip through. Also, maybe some other people rejected the edit but the approvals dwarfed the rejections.
In this case it seems that two edits were done at the same time. The first changed the formatting and removed the obscenity that was present in the original post, and the second only changed the formatting. Because the second edit overrode the first, it looks as if it introduced the profanity, but it didn't.
But I agree, it should never have been approved because it clearly marks the obscene words (you can't tell whether the editor introduced them from that edit alone), and possibly because it is too minor. This would make a good edit review test.