My apology this is what I inferred from reading review of it, on The Verge: "It gets worse with third-party apps. The Pixel C wants to be used in landscape mode, but too many apps assume that they’re on phones. Popular apps like Slack, Twitter, and many more toss you into portrait mode even when the keyboard is attached."

From Ars Technica: "A new entry of late is the "pillarboxed phone app," which is made by designers that see the above issue and say "No problem! We'll put big margins on the left and right of the screen! It's a tablet app now!"