I had the same thought too 
Maybe 0 starts the scale from being "fuckable". So 0 is simply the pass mark that means the girl would in theory get a portion? No need to rate girls that are unfuckable perhaps? But then the 10 scale is still daft. To differentiate between a 7 and a 8? What's that? An alluring mole on the face of one twin?
So when I am driving about, I leave having a punt to chance on occasion and have a competition. In a city it might be £1 for each fuckable bird I see that will give the total amount of cash to spend on a jump. Only see 20 girls I'd bang, I'm hardly going to go looking for a street walker to accept a score for a knee trmpbler. So its 70-100 I'm hoping for minimum. Or it is raining, £10 for one fuckable bird. Or I might go "if I see 20, I will do 30 mins, 30 bird an hour etc". Or other times "See 50, massage and BJ" and 100 the full shabbonk. The mental arrhythmic in keeping honest is tough some times with my cock and balls saying "Go on... only one away and the munter on a bad day would be a hole". So the scoring line is binary - yes or no.
If I had a scale it would be out of 5, and 1 is the "I would be able to fuck that" line. 4 would be the top as no girl is perfect until she got my cock inside one of her orifices. That would cause significant traffic problems if I did that though.
"and now for the traffic with Capital FM. There is a hold up on tower bridge due to a car paused by the bus stop whilst a dirty whoremeister is counting the fuckable ladies there, and is stuck on one which is either a 3 or a 4, and if its a 4 it means he will be booking an hour no hold bars with a prosssie, but if it is a 3, its 15 mins in a walk up so its a critial decision. The traffic police say they appreciate everyones patience, and they hope to get things moving shortly. Now here's Veronica with the weather".