Though I can see the desire for a full length zipper on the Atom and Hydrogen, the warmer bags really don't benefit from it. I have a Helium, Hydrogen and Lithium and never need to vent the warmer bags as they are only used below freezing. Also, what is a "sewn-through baffle" as seen in the Atom description? I thought it was ether sewn or baffled... And I hate the pea green Hydrogen.
Anyway, I am not surprised given how horribly they butchered the design and quality of Dana packs (just had to vent that).
I am guessing that the weight is mostly due to the different fabric as the Helium EQ is the same listed weight (or weighs the 2lbs 5oz that the one I had weighed). That always had the longer zipper, but is using a different fabric. The overall fabric change may be due to problems with acquiring Quantum, or a desire for something more robust for the larger market. Or, maybe it is just cheaper- like the crap materials they used in their Dana packs (more venting). Regardless, I can't see why they changed such a good line of bags.