That thought (about 9+ year average in the Stanford MSTP) had occurred to me as well. Although, I don't think too much should be made of it. Almost every program will have oscillations where people stick around a little longer and then all graduate at once.
I've long thought that programs should not even mention 7 year graduation times (
http://mstp.stanford.edu/structure.html), because it's pretty much an anomaly these days. Finishing is 7 years is a rarity, and for every one that finishes in 7 years there will be one or two who stick around for 9-11 years. Just take a look at some of the student lists out there and it is obvious.
All you entering students out there: Plan on 8 years, and be aware that a small percentage of people take longer. Absolutely do not choose a program because they advertise a shorter graduation time.