Why the sum of zeros and zero is zero

Question

We all know the difference between nulland zero.

ZERO
The value Zero is the value. This is a unique known number of zeros that makes sense in arithmetic and other maths.

Null
"Null" is not a value. This is a placeholder for a data value that is unknown or unspecified. mathematical operations cannot be performed at level zero. Then how is it possible that

[1, null + null] ---> [1,0]
   [1, null + 1] ---> [1,1]

So, here null takes a value of zero as the default value and performs arithmetic operations. Is there any reason for this operation.