It’s traditional for bookmakers to use the odds of Burlington Bertie 100-30 as opposed to 10-3.
In history 100-30 was used in case it was confused for a time (ie 2.50 pm which is 10 mins to 3) rather than a 10-3 price when spoken or signalled.
On-course bookies communicated bets and did not want to mix up the 10/3 prices with the time of ten minutes to 3 oclock.