You say they are the same grain....are they the same brand andmodel BH's? ( I am assuming they are, but you don't specify)
What BH's are they?

Didyou spin test them...both new and old?

While it doesnt seem reasonable, I have seen packs that were just junk. Can you put new blades in the old ones and use them to hunt with? You will need new blades anyway now that they have been shot...or at least touch them up with a stone or small file.

I would take the newest pack back and use the old ones w/new blades. However, you should spin test them to see if they wobble.
I have also seen them mislabled as far as weight.