Why does rebooting give devices different nodes in /dev?

I am using OS X 10.9.4, but this is an issue I've noticed on earlier OS versions. Currently I have an issue with a USB modem hooked up to my iMac (and I have a question that ties in, indirectly with this here), but I've noticed this with other devices.

I have one modem that is set up to receive faxes that is permanently assigned to /dev/cu.usbmodem0000001. It doesn't seem to ever change. When I first plugged in my new modem, it was /dev/cu.usbmodem2. (I had a crappy modem plugged in for a while - I think that was originally /dev/cu.usbmodem1.) I had to unplug it and when I plugged it back in it was /dev/cu.usbmodem3. When I saw this, I wanted to be sure it had changed and it wasn't my memory, so I unplugged it, waited, and plugged it in again and it was /dev/cu.usbmodem4.

I added it to my printers and faxes and thought nothing more about it, but then when I rebooted, my software couldn't find it, so I checked the files and found it as now /dev/cu.usbmodem1.

I would have thought the OS would keep track (using something like UUIDs) of the devices so they can be given the same node each time.

Why does OS X keep changing this around for the same device? Why does my one fax modem (same brand and model as the new one) always have the same device node and this newer modem have one that changes so easily? And is there anything I can do so this modem gets the same node each time it's plugged in or when the computer reboots?

I've asked this same question on Stack Exchange:

http://apple.stackexchange.com/questions/138591/why-do-usb-devices-change-nodes- on-reboot