It should be noted that:
A. The Suns do have the 2nd best 3PT FG defense.
B. Opponents average only 18.6 assists against the Suns (6th best defensively)
Both of these stats are slightly deceiving.
They both deserve to be used to give the Suns credit, but not in the way most people think.
The Suns have a "defensively-sound" offense. Let me try to explain that in a way that makes sense:
The Suns get other teams to play the Suns' type of game.
The Suns drastically increase the tempo and get off quick shots. This often lures the opponent into trying a similar tactic in order to "keep pace" with Phoenix. However, since many other teams aren't used to playing this way, this results in poor shot selection, stupid quick shots in transition (plenty of transition threes), and a lack of sharing the ball on the offensive end (players get the ball and rush up, think they see an opening and try to take it, not thinking about the four other guys who can help get a better shot).
It's incredibly effective and leads to good defensive numbers, but Phoenix's defensive numbers are not all about defense with Phoenix.
It's pretty interesting (and pretty impressive) how Phoenix uses its offense to play defense, if you ask me.