Take Volcano for instance, no Overview however it made it to the top 10 of 2004, or was that a multiplayer mission?
It's a multiplayer mission - no overview.
You're exactly right! Overview and briefing shouldn't be 2/4 of the overall score. This is why they're not. The overall score is not a mathematical average of the four categories (overview, briefing, scripting, camera).
A mission could score for example:
overview: 2
briefing: 4
scripting: 8
camera: 5
overall: 7
Where an average overall would be 5.
So if a mission is great otherwise, but the briefing and overview suck, it can still get good points - just not that good as it would with a good briefing and overview. Usually though, poor quality in the two above lasts through the whole mission.
I have thought up several scoring categories more and in other ways making the reviews more precise, but we won't implement them for the time being.
Read the review. You'll find out exactly how the mission was by reading the whole review instead of focusing on the scores.