Deductions are an industry-wide feature of fixed odds betting, where an amount of cents per dollar bet are subtracted from a winning bet.
This occurs when a betting market needs to be re-framed when horses are scratched from the final field. With fewer runners, your selection is now more likely to win, so you should (usually) receive a lower dividend.
The deduction reflects how much lower this dividend should be. The deduction will be higher when a favourite is scratched compared to when a roughie is scratched,
Say you placed a $10 bet on a horse with fixed odds of $3.00, and there was a scratching which meant a 15c deduction.
To work out your dividend, first multiply $3.00 x (1 - $0.15) = your new odds of $2.55. Then multiply this by your stake to get $25.50.