It’s a common mistake.
Using points per game as the sole determination for who has the best defense has several faults.
1) Lack of Context: This metric doesn't account for the quality of the offenses faced. A defense might have low points allowed because they played weaker offenses, not necessarily because they are a strong defense.
2) The Offense: A defense will allow more points in games if their offense can’t sustain drives, which increases the time the defense is on the field.
3) Turnovers: If a team has a terrible offense (like the Jets), and turns the ball over a lot in their own territory, that sets the defense up to defend fields where the opponents offense is already in scoring range. What does that result in? More points allowed.