Mostly, blades don't weigh the same because of heads, and a few other things. When changing the head on a 206, you balance it to grams, with the blades attached, by adding weight to one blade or the other. Even having the door open while balancing will throw it off. That's just the start, though, because dynamic balance isn't the same as static balance. If it isn't balanced statically, though, you'll have a very hard time getting a dynamic balance. Trying to balance mismatched blades, which have different ages, and different stiffnesses, can be an exercise in futility. There are lots of factors in getting dynamic balance, but it starts with static balance.