As I understand it, when I edit an article, my edits go live immediately.
When I review somebody else's edit and approve it, it still needs more reviewers to approve the same change.
Is there a rationale behind this discrepancy?
Once upon a time, the ability to edit someone else's post was restricted to users with a reputation of 2,000 or higher* (i.e. users who (hopefully) have the experience to use that feature appropriately).
We wanted to unlock the ability for more users to help with editing, so we created the /review system. Any user can "suggest" and edit for a post, but it must be 'reviewed' by at least two users with sufficient reputation to perform that function. This is way more engaging than the comments stating “please sir, edit this for me.” You need two people to do a review, because the review system is more prone to mistakes when the the reviewer isn't really accountable for the content itself.
That's where the discrepancy comes from.
*The reputation requirments to 'edit' change as the site passes from private to public beta and to graduation.