So why does the Arduino IDE connect to an Arduino using a different COM port each time a different one is plugged in, even tho each Arduino is plugged into the same USB connector to the PC ?

This is the case with most USB devices.  The USB transceivers are coded with unique serial numbers, and because of that, the drivers caused them to be assigned a unique COM id.  On my development system at home, I am currently well over COM100.  I have a client that wants to have the USB device enumerate to the same COM number on a system, so I program a fixed Serial Number on the devices, so that any adapter that is connected on a give computer will enumerate to the same COM number.

Can I get some clarification. I find the Windows COM port comedy frustrating. Knowing the cause doesn't solve the problems but at least I have some understanding.

USB transceivers have unique serial numbers S/N. USB COM drivers assign each S/N a COM ID. When a  USB device is detected, the driver deals out COM id's. Each time a new USB device is detected, the assignment begins again and may not remain consistent?

If the device vendor ID, product ID and serial number match then it should in theory get the same virtual COM  port assignment each time, assuming it isn't currently active.

If you type:

   set devmgr_show_nonpresent_devices=1 

from a command prompt before starting Device Manager, then select 'Show hidden devices' 

you should be able to see the virtual COM port assignments for both present and non present USB serial devices.

Some issues may start to appear if you start using multiple devices with the same serial numbers at the same time (or perhaps if the device was still in use by an application when it was removed) as when the device is next plugged in, a new virtual COM port assignment may end up being created. The next time round then you now have two virtual COM port assignments allocated to the same VID/PID/serial number and it will depend on the enumeration order as to which virtual COM port you will be allocated for that device.

A bit of 'housekeeping' in Device Manager can help keep things in order.

Just to add... a useful tool for viewing USB related information all in the one place is 'USBDeview' by NirSoft. It lists all the USB devices and displays extended information including VID PID, serial No, COM port allocation and so on for current and past devices. It might be a rather long list if you are above COM100 though.

https://www.nirsoft.net/utils/usb_devices_view.html

I find this happens with the Arduino Leonardo, the Uno doesn't suffer from this problem.

In part of the programming procedure, Windows see the device do this:

- Hi, I'm a serial device, upload code to me!

- I'm no longer a serial device, I'm in programming mode, but you don't have a driver for me

- Hi, I'm back to being a serial device, upload code to me!

And it's this process which unhinges the COM port allocation. If you can manage to hold the Arduino Leonardo in its 'programmer' phase which Windows doesn't have a driver installed for (yet) then I found it doesn't suffer COM port hopping as badly. I can't remember how to get the Arduino Leonardo to stay in this mode.

Double tap the reset button to force it into bootloader. LED will "breathe" in this mode. Good to recover a ATmega32u4 based board that has a faulty sketch interfering with USB loaded.

- Gough

Double tap the reset button to force it into bootloader. LED will "breathe" in this mode. Good to recover a ATmega32u4 based board that has a faulty sketch interfering with USB loaded.

- Gough