The difference between the Boys not scoring and the Skins scoring a lot is personnel.
This is Romo's 5th season and Phillips 4th. The Cowboys consistently are at the top of the NFL at offense. We pretty much already know what we will get out of them this year in terms of offense. Same with teams like the Colts, Chargers, Patriots, Saints, etc. In that sense, they aren't going to throw a lot out there. The offense looked solid as usual with the exception of scoring in the redzone.
The Skins have a new coach and new quarterback so they have to get as much reps as they can before the season starts so this will lead to their starters being in longer, more wrinkles, and more scoring.
From what I saw from McNabb and Co., this might be the best Skins offense in a very long time. I think they will turn a lot of heads this year.