For what it's worth.
Mac's that are brand new like the latest models cannot have an OS installed lower than what is shipped. However, a model like the one you and I both have (Mid 2012 MacBook Pro)
CAN have an OS installed lower than what was shipped.
The shipped OS only applies to brand new models. And I mean brand spankin' new
I can confirm this, when my work place bought a refurb Mid 2012 13'' i7 MacBook Pro that came with El Capitan, we were able to wipe the drive and install a fresh copy of Mavericks