A)It's actually something to do with all models and the replacements just aren't used long enough to be worn out
B)They just learn to make them more durable as time goes on
C)They deliberately go with flimsier parts for launch models to cut costs, then go for more durable parts later on as the costs of other parts of the console go down.
I would guess that it's a combination of all three.
Regardless, you certainly can't compare it to the failure rate/average lifetime of the 360.