For all the numbers from 10-99, when I add their digits and then subtract the sum from the original number, I always get a difference that is a multiple of 9. For example, for numbers 20-29, the difference is 18, for numbers from 30-39 the difference is 27, for numbers 40-49 the difference is 36 etc. No matter, what number you pick, the (original number - sum of its digits) is a multiple of 9.
The Number Jumbler Trick
In every round, the Jumbler displays the same symbol for every multiple of 9, sometimes it shows a bike or a cherry or a cat or an engine or a house for all multiples of 9. Every time you use the jumbler, it changes the symbol for the multiples of 9. The reason why the user gets tricked is because the Jumbler uses a different symbol for multiples of 9 for every round.
So, it is maths not magic.