So, why change the DevKit to 8GB from 6GB?
The current Switch uses 2x 2GB Samsung memory modules. The Dev Kits, obviously, had 3x. So Nintendo was able to just focus on buying 2GB modules and maximized their volume discount.
My bet is they've switched the Lite and the "new" revised Switch to use 1x 4GB module. With density increases etc, the prices on modules drop over time, so it's probably now cheaper than 2 smaller modules. That leaves them with either: continuing to buy a vastly reduced amount of 2GB modules for the additional DevKit RAM, but now they're paying a lot more per 2GB module because they build a lot of DevKits, OR just buy a ton of 4GB modules, maximize their volume discount, put 1 in consumer Switches & 2 in the DevKits. No dev ever complained about more RAM anyways.