Just what is the "public Internet"?

From the official Comcast page justifying their bandwidth caps (emphasis mine):

Comcast has set a data consumption threshold (“threshold”) on monthly bandwidth consumption (including both upstream and downstream usage) by residential users of its high-speed Internet service (“XFINITY Internet”). Currently, that threshold allows a residential customer to send or receive up to 250 Gigabytes (GB) of data in a calendar month. This includes data in any form (including movies, photos, music, videos, e-mails, computer back-ups, or other types of files) that a customer uses his or her XFINITY Internet to send or receive over the public Internet, including data sent by one XFINITY Internet customer to another.

So a FaceTime call from my house to my neighbor’s—which never leaves even the SF metro area Comcast network, given that both of us are Comcast customers—goes over the “public Internet.”

Yet Comcast’s Xbox streams, which pass from Seattle to Sacramento to San Francisco through all of the same network elements that handle my video call (and then some!) are exempt from the bandwidth cap?

You can’t have it both ways, guys.