Maybe the difference is in the maintenance that is required.
For example, maybe the older down garments/equipment suffered less loft degradation due to body oils/sweat. Therefore, you never laundered them.
I bought a department store down quilt several years ago for $40 on sale. After two or three years of use of over 100 nights per year (200 to 300 nights total), the loft was considerably less. I washed it with down specific soap and dried it in a commercial large dryer following instructions for high quality down gear. The loft gain was astonishing.
I used it for two more years, and then became bored with it. So, I bought a new one (on end of year sale) with 550 cu. in. per oz. claimed fill.
I have a 700+ down fill bag that suffered from loft loss and smell pickup from a hot summer on the Appalachian Trail. That laundered up real well. In fact, I used the down quilt as practice before I did the bag.
There is a higher feather content in the typical lower loft "down" products. Feathers are expected to degrade in loft differently than the fluffier down clusters that dominate in the high loft "down" products.
It is common to claim that the traditional backpacking products were made "bombproof" while the new lighter weight products require more care and careful handling as part of the tradeoff between performance and weight.