Does anyone know why you get so many more points for winning a game of original than you do arcade or regular?

i think cos originals harder but im not sure

The ranking system is explained in the help:
http://www.funkysnooker.com/help/rankings

At the end of the 1st section there's a table with the "Ranking Scale": that's the one which makes a difference between game types.

As for why original's ranking scale is higher, I'd say it's because original games last longer than arcade and regular, so you get as many points for your time.