Yes, the only hard rule in my department is having a minimum of 50 pages, the idea that 100 pages is not enough came from the scientists applying their own experiences from years ago. Technically there was nothing they could do about her thesis having fewer pages, but as inexperienced students, it's obviously a little scary when people you look up to sound concerned (since academia is full of all sorts of unproductive and unstated expectations).

A friend at another department only had a minimum requirement of 5 pages, and his thesis ended up being just a collection of his publications.