The sound wasn't 5.1 from NFL Network, so the 5.1 you were seeing via OTA was really 2.0 labelled as 5.1.
Interesting, NFL net put my HU into PLIIx mode and nbc and cbs were DD5.1. CBS was coming out of the front center only and NBC was coming out of the front L&R which sounded better and made me think it was surround and not stereo. The NFL net sounded garbled out of the surround speakers.