Except for three games, the Cowboys and Eagles had the same schedule. In those three difference games - Dallas had Tampa Bay/Cincinnati/LA Rams, Philadelphia had New Orleans/Pittsburgh/Arizona - both teams were 2-1, so it was a wash, it made no difference to the final standings.
The Cowboys and Eagles played each other twice and both teams were 1-1, so again it was a wash and made no difference to the final standings.
The remaining 12 games on the schedule were against the exact same teams. The Cowboys were 9-3 and the Eagles were 11-1. That was the difference between the #5 seed and the #1 seed.
The NFC East teams played the entire AFC South, a weak division. Outside of the NFC East, the entire NFC was weak with only a few good teams, leading to an "easy" schedule for both the Cowboys and Eagles when playing teams outside of the NFC East.