Two different cats

I consider myself fortunate to share my home with two cats, companions both for each other and for resident humans.  Particularly given that they're related to one another and from the same origin, and have spent almost all of their lives in the same territory, I'm intrigued to note how they've developed dramatically different personalities (felinalities?).  Partly, this seems to reflect human gender stereotypes.  Our male is larger, more assertive, and much more vocal.  Our female is quiet, reserved, and meows delicately and selectively on occasion.  She'll consent to his attempts to groom her, but sometimes with apparent reluctance.  Still, it's clear that they've established a close feline relationship, partly based on communication not fully apparent to the humans.  It may be imponderable just what to attribute to nature vs. nurture vs. developmental history, but I'll ponder it anyway.  Perhaps evolution has selected in favor of a randomness trait, so different individuals within a species won't generally become identical, thus allowing them to complement rather than duplicate one another?