The simple answer on why you have two different length pushrods is ANGLE. The intakes are offset and lean over more to get around the intake port. You lean it over, it has to be longer to cover same distance.

Many motors have 3 different lengths depending on port layout. One exhaust length, a left intake and a right intake length

ALL pushrods flex, nearly impossible to stop it. We have 9/16 intakes and 5/8 exhaust in our motor......they flex. Of course we have 500lbs on the seat and a cam with 1.200 lift