Since we find that the bechorah went to Joseph, and the bechorah is for all generations, then just as we find that the bechorah for Joseph was a double portion of one (son), so, the bechorah for all generations is a double portion of one.
"a double portion" in all that is found with him": We are hereby taught the bechor does not take a double portion of what is anticipated as he does of what is on hand.
"for he is the first of his (the father's) strength": and (the criterion for bechorah is) not the first of her (the mother's) strength.