The NI-Serial driver has functionality that allows it to keep track of serial port properties, including COM port numbers. For example, say there are two USB-232 devices and each port of each device is connected to a different instrument. Each instrument has different serial properties. So each serial port will have to have these properties.
The Windows registry stores this information by the serial number of the USB device and the port number. This way, each time you plug a given USB-232 into that computer, it will retain the selected properties. Thanks to this feature, you do not have to configure the port every time you plug in the device. A side effect of this functionality is that every new device must be assigned to an entirely new COM port, one which had never been used on that computer.