OT: Deciphering NFL broadcast rules

Can someone explain the how CBS and Fox determine what games to broadcast in a given location? I live in Chicago. I HATE the Bears (Vikings fan since Joe Kapp played). I thought I had an idea of what I could expect to see when the Bears are home and when they're away. But, yesterday made NO sense. The Bears were @ Seattle on Fox. So, we got no early game on Fox but did get early and late games on CBS. What games did we get? Early: KC v Philly, Late: Pitt v Cinncy. WTF! There seems to be a lot of Packer fans in Chicago-land (which is understandable) and we get Packer broadcasts for the most part (GB was @ Rams yesterday). But, those two CBS games made ZERO sense to me. I've been a DirecTV subscriber for quite some time now, perhaps I'll shame them into giving me NFL Sunday Ticket for the rest of the season for free. I ended up watching the Vikings game on-line (and am glad I did!) So, if anyone can shed a little light on how things work, I would truly appreciate it.