Mountain Lion does not show up under Purchased for ML pre-installed machine
If Mountain Lion is pre-installed on your Mac, you won't normally get a redemption code to purchase the "upgrade" but that serial number will be checked if you have to do a bare install, and the installer will download because the Mac deserves to get Mountain Lion internet recovery install on a bare drive.

Basically, the license is tied with the hardware, not the Apple ID. However, some Macs purchased during the window surrounding the release can often take advantage of the Up To Date program which according to the web page is intended for Machines that shipped with Lion and not Mountain Lion.

The upgrade lets you install the software on all of your personal Macs whereas the machine license generally lets you just install it on the Mac that shipped with the OS.