Seattle's defense is very bad, and was also very banged up in the back end , so Bills took advantage of that.
Josh and the offense still needed to do it, and they did.
All of this really boosts the the thought that every week is a new week, teams go from healthy to banged up and back, and that game predictions are fairly useless because past roster performance is a lousy predictor of future roster performance as the roster actually on the field can change drastically from month to month.