Part of it is because of the divisions the AFC East faces this year. If you notice, every team from our division is in the top 7 for strength of schedule. The Jets have an easier schedule than the Bills because they play each other twice, and the Bills had a worse record last year.
If you notice, the Dolphins supposedly have the most difficult schedule. They had the same record as the Pats, only they won the tiebreaker...so all things being equal, their schedule should be harder.
The Bills had one of the hardest if not the hardest schedule for years. Sometimes bad teams end up with the hardest schedules because they are in a good division, where they have to play every team twice.
Strength of schedule based on the previous year isn't an exactly air-tight evaluation on who has the hardest schedule anyway. I'd rather wait until the end of the year to judge how hard a team's schedule is. Good teams from last year will be bad this year...bad teams from last year will be good, etc.