How does null pass into a factory's method signature with an optional param and default value?

main.dart

void main() {
  Map<String, dynamic> json = {'type': null};

  try {
    ElementType too = ElementType.fromString(null); // passes null through (explicitly)
    ElementType foo = ElementType.fromString(json['type']); // passes null through (explicitly via map)
    ElementType bar = ElementType.fromString(json['key_missing']); // passes null through (implicitly via map)
    ElementType baz = ElementType.fromString(); // does NOT pass null; factory uses 'default string'
    
    HashyType bro = HashyType.fromString(stringValue: null); // passes null through (explicitly)
    HashyType was = HashyType.fromString(stringValue: json['type']); // passes null through (explicitly via map)
    HashyType sup = HashyType.fromString(stringValue: json['key_missing']); // passes null through (implicitly via map)
    HashyType dud = HashyType.fromString(); // does NOT pass null; factory uses 'default string'
    
  } catch (e) {
    print("Error: $e");
  }
}

enum ElementType {
  audio;

  factory ElementType.fromString([String? stringValue = 'default string']) {
    print("String value passed through: $stringValue");
    return audio;
  }
}

enum HashyType {
  bar;
  
  factory HashyType.fromString({String? stringValue = 'default string'}) {
    print("String value passed through: $stringValue");
    return bar;
  }
}