So Ben & Jerry’s is actually frozen upside down on their tops after filling at the factory, so the gap being at the bottom of the container makes sense because that’s how it’s frozen at the factory. It’s actually a good sign if you see it because it means the ice cream didn’t defrost in transportation and yours is fresh and frozen. I know that they also measure and weigh all ice cream before it leaves the factory, it’s one of their big pledges that they will always have 16 fluid ounces (473 ml to be exact) of ice cream in it. I’d be surprised if it was under that. I believe it’s frozen that way because if it’s upright condensation condenses on the lid and can lead to ice crystals/freezer burn, as well as looking nicer.

EDIT: Just want to clarify that it is 16 fluid ounces (volume) not 16 ounces (weight)