Comment by petschge

Nope, they are quite different exactly because "something you are" is attached to you and "something you have" is not. One can be swapped out if compromised or get lost. The other can not (intentionally or unintentionally) be replaced, but -- because it is something biological -- undergoes slow changes over time. These differences are sufficiently large that it makes sense to split it into two categories when modeling the whole system from a security -- or usability -- standpoint.