I have never fed my dogs by the guidelines on the bags, I've always found that each dog is too much of an individual for that. I never really went by weight either, because 70 LBs on my dobie is not the same as 70 LBs on my dane/IW mix. As long as their coats are good, muscles developing normally, energy level high and eyes bright and happy, I wouldn't worry. You should be able to just see (or feel, in heavy coated dogs) their ribs, as too much weight on a growing puppy can lead to alot of joint and bone issues. Also, pups will go through growth spurts, and their needs will fluctuate to accomodate that as well
Merlin-4 years old eats 10 cups of food plus 1 can a day, he is thin as a rail, but healthy as a horse per my vet.
Raven 11 years old eats 3 cups of food and 1/2 a can and is a little fatso.
Strider is 11 months old and eats 6 cups of food and 1/2 a can and is ideal body form and condition.
As you see the amount needed can really vary.
Stella