It's like RAW (rules as written) vs RAI (rules as intended), the definition of furry is an individual who is especially interested in anthropomorphic or cartoon animals. They often strongly identify with anthropomorphic animals and create fursonas, identities of themselves as those anthropomorphic animals. While, yes, humans are animals, the average English speaker will interpret it as non-human animals. Also, even then, it is associated with being enthusiasts of anthropomorphized animals.