Just because terrible teams sometimes beat good teams doesn't mean they're not bad teams.
The Bills are solid, but let's not pretend that the AFC East hasn't been one of the weaker divisions in football for years. Or that the Bills would have an 8-3 record this season playing in 3 other divisions. Between seeing the Jets, and Dolphins twice, and then getting to play the Giants plus Bengals to boot...you gotta really try to come out with a bad record.
It...hasn't really, though. Since realignment (2002), the AFC East has the best winning percentage of any division in football. And even when you remove every division's winningest team over that timespan (controlling for the Patriots), the AFC East still posts the fourth best winning percentage of any division. Not amazing, but still in the top 50% of divisions.
If you want to know why the rest of the East looks bad consistently, it's usually because they're in the same division as New England. Not always the case, obviously (last year's AFC East was by and large an unmitigated disaster), but the division is nowhere near as bad as people make it out to be.