It probably is both ;-) (I usually say that burn-in == brain adapting to new signature).
I remember talking to one of my friends at work telling him that at first the sound might even be underwhelming... he agreed... But after a few songs in, he wanted to take the headphones with him! LOL... HELLSSSS NO!!
I also provided both my Sig Pros and the MDs while he was at my cube. He liked the MDs more than the Sig Pros based on comfort and a "non-agressive" approach as he called it. I don't particularly see the Sig Pros as agressive (listening to Moving Mountains now... oh so sweet! ;-)) per-se but they are indeed bassier and have a bit more detail up top.
Still, with the MDs there's definitely an adaptation time. I'm pretty sure if you jump back to any of those other headphones/IEMs and then come back again, the same thing will happen! ;-)
I think this is definitely a real (if not THE real) phenomenon, and furthermore, the longer you "burn-in" a particular set of headphones, say the MDs, the quicker your brain can re-adapt to it as you cycle through your collection, to the point of almost instantly.