I am waitlisted HST as well and was wary of those numbers. However, I find it very hard to believe that "very few" people get off the HST waitlist. For one thing, I know that a number of people will choose between HST-MD and some non-Harvard MSTP program. While Harvard is Harvard, it would certainly be hard to turn down 8 years of funding at another great school.
This is purely a guess but I wouldn't be surprised if they don't take about 10-15 people off the waitlist (of 25), which accounts for the percentage they gave. I would bet they accept 40 people for those 30 spots, but in general only about 20 of them matriculate.
...Heres to wishful thinking =)