What "Pixel" denotes is "cargo cult dogfooding." It's when you sell your customers dry kibble dog food and you have some high-end dogfood around for your engineers, some of the canned stuff that you let the really clever dogs cover in veal demi-glace through Crouton. (Yes, I understand that I'm comparing Ubuntu to something top of the line, it's not a perfect metaphor.)
Google, outside of their smartphones, makes computer products for children. I'm serious about this, I'm not being condescending to Google users, I mean actual children. Android tablets are the tablets you give to a four-year-old to occupy them on a long car trip or while you're making dinner. They're small enough for a child to hold comfortably and they're cheap enough for you to not stress out what the child is going to do with them. Chromebooks have caught on in exactly one market, education, because children can be real assholes and giving them a computer that can't actually run software eliminates a lot of the headaches that come from giving Turing-complete devices to real assholes.
But Google engineers aren't children. They want real computers. What they probably really want are MacBooks, just like everyone else. But MacBooks aren't dogfood. The Chromebook Pixel exists pretty much entirely so that Google can claim to dogfood ChromeOS and engineers can get a laptop that works for them, if you consider putting up with Ubuntu over OS X working for them. It's all an elaborate charade, in that running a high-end Chromebook Pixel and using Ubuntu through Crouton or just wiping the thing has about as much in common as the $200 Chromebooks that most ChromeOS users actually have as just giving the Google engineers their MacBooks, but Google seems to regard dogfooding as something more honored in the breach than in the observance, so that's just par for the course.
So why is this a Pixel and not a Nexus device? Well one, because Google makes it themselves. But why are they making this themselves? Or at all? Because Google engineers want a Surface or iPad Pro. And people who work for Google are about the only market for a Surface or iPad Pro knockoff running Android, because Android is such a terrible tablet ecosystem for anyone who has app needs beyond Netflix and Toca Pet Doctor and I don't see this thing moving the needle on that point. And since it's running Android and not ChromeOS you can't even get any of that Ubuntu demi-glace on it. So even by the standards of the Pixel line this is such an awkward compromise between dogfood and something edible.