C++ Screen position gets weird when I try to convert from Getcursorpos to Sendinputs function

Question

I used the function GetCursorPos to pick up positions on the screen I would like the mouse to be able to click to perform a task for me and automate the service.

I tested selecting these positions with the SetCursorPos and it worked, however the program using needs to be fullscreen and function SetCursorPos does not work, so I used the function SendInput but the same coordinates seem different.

Follow the code from the SendInput:

inline bool mouse_move(int x, int y)
{
    INPUT input;
    input.type = INPUT_MOUSE;
    input.mi.mouseData = 0;
    input.mi.time = 0;
    input.mi.dx = (x + 1) * (65535 / GetSystemMetrics(SM_CXSCREEN));//x being coord in pixels
    input.mi.dy = (y + 1) * (65535 / GetSystemMetrics(SM_CYSCREEN));//y being coord in pixels
    input.mi.dwFlags = MOUSEEVENTF_ABSOLUTE | MOUSEEVENTF_LEFTDOWN | MOUSEEVENTF_LEFTUP;
    SendInput(1, &input, sizeof(input));
    return true;
}

Follows the function I got the positions on the screen.

POINT P;
GetCursorPos(&P);

Clearly this requires some kind of coordinate conversion but I cannot accomplish such a thing.