The Precambrian Rabbit

Stanford Genetics!
Anyone else going to Stanford?

Stanford Genetics!
Anyone else going to Stanford?

Stanford Genetics!
Anyone else going to Stanford?

Stanford Genetics!
Anyone else going to Stanford?

Stanford Genetics!
Anyone else going to Stanford?

Harvard Systems Biology

Harvard BBS!

To answer your question, I'll be honest and I'm sorry if it's a little harsh. But I hope the info from the directness is worth it.
In addition to what Bioenchilada said, "not a good fit" is also a way for schools to reject you without having to say exactly why, to avoid hurting people's feelings and to avoid an argument with an applicant.
Also, it's likely that the first cut (interviews) were made with little regard for "fit". They might choose to interview the most promising applicants and then decide later whether or not there is room for them in the department.
Finally, "fit" is a nebulous and subjective thing and I've learned that whenever schools say no for reason X, it doesn't always mean that "X" is the ultimate reason for saying no. In this case, it might not be that your interests aren't a good fit with the department, but the department only has so many spots for people with an interest in X and maybe they decided to admit the other people instead of you. What they said is still true, because now the department "needs" are already filled, so your interests are no longer a good fit for what they are looking for. (Note: I've seen this applied to other things too, a prof might say "I don't have funding for another student this year" but they might really mean, "I'm limited in my funding so I'm waiting for a really exceptional student" etc.)
In any case, I wouldn't worry about it---grad admissions are tough and a lot of the times, the decisions depend on factors you can't control, like who else is applying and what the department specifically wanted this year. It looks like you have at least one admission, good luck with the others!

I think they were just trying to tell you that the applicant pool was competitive and nothing personal.

I'm committed too! So see ya'll this fall

I actually found some data on HILS website and did some math based on the data. The result confirms what @Epigenetics just said:
(source: https://gsas.harvard.edu/programs-of-study/divisions/harvard-integrated-life-sciences)
(1)Last year all programs in HILS (BBS, BIG, MCO, immunology, chem bio, etc) accepted~396 students (2331 total applicants*17% admission rate), and the entering class is 210, which makes the yield rate ~53%.
(2)Since BBS is the largest cohort in HILS (~31% of the HILS), let's assume that the yield rate of BBS is similar to that number of the whole HILS. 
(3)In order to fill 65 spots, BBS needs to accept 65/0.53=122 students. If the # of spots to fill is 70, they need to accept 132.
(4)~120 people will attend two interview weekends, and there are more internationals do Skype interview. Harvard BBS has ~30% intl students, let's assume that among 30%, half (15%) reside in the US and are already included in the 120, and the remaining 15% will do skype. The total interviewee number (onsite+skype)=120/(1-0.15)=142. 
(5)The conclusion: BBS will interview ~140 students and accept 120-130 students, which makes the post-interview acceptance rate 86-93%. Not bad at all.
So guess we can just chill

I've heard that Princeton Molbio will finalize their result next week. Good Luck!

Congrats! I got it as well.

All my friends (including myself) didn't take subject GREs, and we all got to top programs. I was also a little bit worried because most programs explicitly say that "subject GREs are highly encouraged), but at the end I think it doesn't really matter.

All my friends (including myself) didn't take subject GREs, and we all got to top programs. I was also a little bit worried because most programs explicitly say that "subject GREs are highly encouraged), but at the end I think it doesn't really matter.

Congrats! I got it as well.