Help with PHRF rating question for Scouts

[Quest]

We're working with several of the local Sea Scouts and some of the kids have a question that I can't come up with a good answer for.

We've been racing non-spin for now and the kids are wanting to learn how the rating system works.  Getting the basics of TOD comparisons were easy and then they started asking why the ratings are what they are.

Now my dumb luck, we're racing against the 1 tonner.  She's being sailed by good sailors who are good people and the certainly don't make many mistakes.

So to their question is how she gets the 140s rating she has.  My initial answer was there's a formula that gives a base rating and then the local board members take local conditions into account and derive a base number.  Then the teenager asks that wonderful question "Why/How".

This page at Brown University was made known to me and after reading it (especially the last paragraph), I'll admit I'm over my head on this answer. 

http://cs.brown.edu/~jfh/boats/FAQ/node13.html

I'm not trying to start a fight, I'm just trying to give them good explanation.

[Bee]

This is supposedly how PHRFGB does it

http://www.phrfgb.com/ratingprocess.htm