It's not weird 
Each time you reload the page, the symbols are regenerated, but always in a specific way to trick you.
Have you noticed that if you choose a number between 30 and 39, the final result will always be 27 ? Same goes for 40 to 49, etc...
The maths behind that are simple. First you get to choose a number c = 10a + b.
Then if d is the final number, d = c - a - b = 10a + b - a - b = 9a. So whatever the number you choose, the final result is always a multiple of 9. Then it's quite easy to generate the same symbol for all these numbers.
If you look at the table, the symbols for 9, 18, 27, 36 etc are always the same. Got it ? 
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If wishes were fishes, we'd all cast nets.