why and how is there ANY material difference between the two???
For JimJ, the material difference is the bootloader (more specifically, the baud rate). Like JimJ, I would assume a board labeled "Uno" would use the "Uno" selection when uploading. If they had installed the Uno (optiboot) bootloader, JimJ would not have had this problem. A trivial and obvious choice for OSEPP with an annoying consequence for JimJ.
As far as I know, there are three major Arduino bootloaders...
"Old" /Duemilanove bootloader - STK500 protocol; some variation in baud rates (e.g. lower rate for wireless boards)
"New" / Uno bootloader - named "optiboot"; STK500 protocol; 115200 baud; much smaller than the "old" bootloader leaving more space for the user's application
Mega bootloader - STK500v2 protocol; lots of built-in stuff (like a monitor)
Some details about the various bootloaders can be gleaned from the boards.txt file located with the Arduino software. Any additional details you'd like to know can be gleaned from the bootloader source code also located with the Arduino software.