不可变的字典去深复制为什么变成可变的

        NSDictionary *d=[NSDictionary dictionaryWithObject:@"aa" forKey:@"AA"];
        
        NSDictionary *d1=[d mutableCopy];
        [d1 setValue:@"1" forKey:@"2"];
        NSLog(@"%p",d);
        NSLog(@"%p",d1);