serial from usb port, cu using /dev/cuau0

Connected a USB to serial dongle to a USB port,
then a straight-through serial cable to my dsl modem.

as root:
   cu -h -t -s 9600 -l /dev/cuau0
or maybe cuaU0

Whether anything is connected to the USB port or not,
when I issue the cu command, I always get a "connected" response.

On one occasion two days ago, I swear the cu connected to the
usb-serial and I got a login prompt and poked around the modem.

Assuming that actually happened, how does the system know which
usb / serial port to use?

This mobo has a serial port with a dongle connected.  It appears
to be activated in BIOS, as when I direct connect the serial cable
to it I get a connection to the modem.

But I'm confused as to how I got it to work from a usb port.
Thanks for any insights,

Gary