That did cross my mind.....
The ability to Boot into 4K@60hz (DP 1.2) happened on the 2012 machine as a consequence of applying the Sierra upgrade over a Yosemite setup. This same process did not have the same effect on the 2010 machines.
Taking the boot drive out of the 2012 cMP and putting it in the 2010 cMP does NOT enable the 2010 to boot via DP1.2 - it hangs as before.
Left me wondering whether there is something in the 2012 machine's 'ID' that enables system functionality that doesn't occur with the 2010s?
Interestingly, this 'ability' is initiated (or not), almost immediately after the chime - so very little System software loaded at that point (if any).
DID however notice that resetting the PRAM (NVRAM) on the 2012 machine STOPPED it being able to boot into 4k@60Hz... it could only manage 4K@30Hz (like the 2010s) !!......had to restore it from its TM backup to get the 4K@60Hz functionality back......
The clue to what's going on seems to be embedded somewhere in the above.