Not sure about diversion rates but it is notable in this part of the world that flights to some destinations are weighted with passengers who are traveling for medical reasons.
Recently caught a flight Hong Kong - Vancouver (CX 777).
In Vancouver, I counted 23 wheel chair passengers alighting.
That sort of thing might skew the diversion statistics but unlikely mid-Pacific or over the Aleutians.
Oh yeah, and oysters in BKK before a flight to Perth was a really bad decision on my part.