There is an option when reviewing an edit to improve the edit before approving it. This is useful for correcting a minor mistake or clarifying something before submitting the edit. There is a check box that says something to the effect that the edit was useful before the reviewer's improvement. If this is checked (as it is by default) then the original editor's changes as well as the reviewer's changes show up in the edit history.
I'm not even sure that users who can edit without review even gain reputation points for editing a question or answer. I edit things fairly frequently and I can't remember the last time I gained any reputation points for doing an edit. I think you only gain any points if your edit is approved by a reviewer.
The other thing is that if a user who has 'edit without review' privileges does their own edit the change appears immediately. If they are the first to approve another user's edit the changes do not appear until another user also approves the edit. This edit could languish for hours, days, weeks, or even forever if another user never approves the edit. Once an 'edit without review' user has approved another's edit, it seems that they are locked out of making any of their own edits to the post. Sometimes the user with 'edit without review' ability may feel strongly enough that the edit needs to be enacted immediately and will do their own separate edit for that reason. As pointed out above, no reputation points are awarded when an edit does not need to be reviewed by other users.