Facebook has all kinds of algorithms that they use for friend suggestions. Could be based on some similar interests, likes, locations, work, etc. Could be on other details. Mutual friends is just but one aspect of it (perhaps a more common/important one at times, but not necessarily all of even most of it).
Aside from that, they are always working on various improvements and changes to their algorithms (similar to features), so there are almost always some people that might have suggestions where one aspect (like similar locations or likes) plays much more of a role than any other aspects, or something else of that nature.