Here, OXM sets it clear!
http://www.oxm.co.uk/67538/nuclear-throne-announced-for-xbox-one-idxbox-day-one-parity-clause-revealed/
I quote:
"Ismail also shed a bit of light to Joystiq on the inner workings of ID@Xbox - apparently, Microsoft insists that developers release their games on Xbox at the same time as on other platforms, though there's no such restriction on games that are already announced for other platforms (hence the confirmation yesterday that PS timed console exclusives Divekick and Cosmic Star Heroine will release on Xbox One).
Numbering just two people, Vlambeer is ill-equipped to develop and release a game for multiple platforms at once. Accordingly, it got in touch with Sony - which doesn't insist on release parity - to secure a PS4 deal first, and escape the simultaneous release requirement.".
This, I stole from grandpa...It's called soap.
