Welcome to the Make community!
This is because Make uses JavaScript (TS?) on the backend, and it is exactly how JavaScript (programming language) will treat those variable types.
If you don’t believe me, just open up your browser console (which also uses JS) and type these in:
Numeric + String = String
Numeric + Numeric = Numeric
Numeric - (Numeric or String) = Numeric
JavaScript uses the + operator for both Math and concatenating Strings. So when any of the variables are strings, the output is concatenated instead of summed up.
JavaScript doesn’t use the - operator for anything other than Math, so the values are treated as Numeric. This is called Type Coercion.
What happens if you subtract a non-numeric value?
You get NaN (not a number), basically an error.
For more information about Type Coercion on Make, see https://www.make.com/en/help/mapping/type-coercion