Actually, you helped explain what happened to me. I tuned into WCBS after 11-pm to check the news and was dismayed at hearing Monday Night football featuring two out-of-town teams. Thankfully, that's not going to happen every Monday night all season.
Apparently, and this is just an educated guess, WFAN is the regular Monday night Dial Global network affiliate in NYC for Monday Night Football, but until the end of the baseball season Mets games come first on The Fan.
So, to keep the revenue in the house CBS moved Monday Night Football to WCBS-AM, and as soon as the Mets game was over WFAN picked it up too. So, WCBS picked up the game from the get-go, and WFAN also took the network feed when it could.