Peculiar CGA-S006 from Battery Barn

[Lens Flare]

I recently got a new CGA-S006 from Battery Barn which in the past, from the case and weight, is probably something like two 18500 cells in a plastic case with some electronics added. However this one felt completely different, it feels as if there are much smaller cells in there than with previous ones I'd got from Battery Barn (and the Panasonic original), and the whole pack weighs about 20% less than the earlier ones I've got (without cutting it open I'm also not sure what's really in there, so the 20% may be a lower limit on the total loss in cell mass). I realise power densities are going up slowly over time, but that'd be quite a leap. Has anyone else noticed such a difference in (supposedly identical) batteries? Both of the BB batteries are their Power2000 brand, bought about two years apart.