There is no conspiracy theory involved here. There is no WP overlord deciding which devices shall be blessed with updates first. Least of all, it has nothing to do with the 8X being a "signature" phone. Reading anything into this is a complete waste of time. Reality is much more mundane. HTC was simply ready earlier.
Nokia tells me they have WP updates from Microsoft that are ready for deployment to their devices, but they also have other issues they would like to address with the same update. The overhead involved in distributing an update is very large, so distributing two smaller updates requires much more effort than distributing one larger one. Possibly HTC didn't have anything else they needed to address and just deployed what they got from Microsoft. I don't know, but whatever the reason is, it isn't anything more meaningful than that.