The zero comes in handy. Is it a number? Come on.
10-7 = 3
If you have 7 succeeding eight in the horizontal column you can have only 3 below the eight.
7+3 = 10.
In the second horizontal column you have 8 behind the 3. You can have only 2 under the 3.
2+8 = 10.
And so on and so on until the sequence is cut by
11111111. The end of the triangle.
The reflection gives only back what is.