"Crackle" glass is made by briefly dipping hot glass into water. This results in surface fractures that are easily detectible by touch, even if the glass is "healed" by reheating in the glory hole. Internal fractures are the result the item being placed in the annealer too cold, an inadequate annealing cycle, or incompatibilities in the glasses used within the piece. If the fractures seem to run along side a bit of color then incompatibility is the most likely answer. If the fractures are essentially flat planes within the center of the weight, then it is most likely the piece was put away too cold and the annealing process was unsuccessful for that reason.
I too have been told by the seller that internal cracks are a design feature intended by the maker of the weight. That story is pure BS.
It would be possible to crackle a small gather then encase that in additional glass. This would result in a weight where small cracks would be visible running in a thin layer around the interior of the weight. It would not result in flat planes radiating from the center of the weight.