I wish i understood it too but frankly i don't know how it's possible either. My brother and i both had rats, i had boys and he had girls, and we are both react more to the does. For me it's not as bad, if either doe or buck scratches me it gets itchy, red, and swollen but if i handle a male rat and bury my face and nose in their fur and breath in I'm fine, but for some strange reason if i do this with a female rat my nose gets a little snuffly and my face itches slightly. My brother is a little more sensitive to allergies like with cats and yes rats too but he reacts a little worse to his females than to my males. I'm sorry i know it makes no sense whatsoever, especially since all the rats were related as mother and her babies, but we tried with an unrelated buck of mine and got the same result.