Why in racing odds do they use 100-30 and not 10-3?

Question

You don't say 20-10 but 2-1, not 90-20 but 9-2.

So why not cancel 100-30 down to 10-3?

Answer 1

It's related to the gestures & slang used in tic-tac (the secret code bookies use to indicate the changing prices of a horse):

9 -2 - or 'shoulders' -  get 4 and a half times the stake for a win.

Burlington Bertie 100-30

or

scruffy and dirty, one hundred to thirty.

Yes it all works out the same - the actual terms are just used like that because of the rhyming slang.

You asked why not cancel 100-30 down to 10-3 it's because the tic tac rhyming slang does not match 10-3. It's just like that - it's like asking why don't we change the dictionary around after all these years.