I am pretty sure that the ALT key keycodes are generated by the HID (and pedantically, it’s not a HID device, that’s like an ATM machine 
). So holding ALT from a different keyboard and pressing numbers on the keypad is different to holding ALT and pressing numbers from the one device. I can’t recall where I saw that documented long long ago, but I remember thinking it mist have been accurate (it may have been on some legacy IBM website)