"Current Version" is not an edit entry, in itself, it's just there so you can include it in the comparison if you need to. Each other line represents an edit, when it was edited and who made that edit.
No, the current version is different than Greg's entry. If you compare current version with Greg's edit, you see Greg's edit, if you compare Greg's edit with mfb's edit below you see mfb's edit. It's like shifted down.
The basis for comparison for each of the edits is the pre-edit version. So in your example you're comparing the current version (which was created when Greg edited the post) with the pre-edit version of that edit (which was created when mfb edited his post).
I still think the times and user names are wrong by one row. Alternatively, the radio buttons are wrong by one row. If I compare the version "Mar 24 8:42, 2017 Greg Bernhardt" with the earlier version "Mar 24 8:31, 2017 mfb", I would expect to see the change Greg made on March 24, 8:42. What I actually see is mfb's earlier edit: the change mfb made 8:31 compared to the version of Mar 24 8:27. But that's not what I selected. Am I just being daft here?
I can see how it's potentially a little unclear, though what you're seeing is mostly how the system is designed. The stored version reflects before that person's edit, so to see what a person changed you select that entry and the subsequent one. Essentially, the values indicate the version of the post up until the date of the edit.