A lot of times it's cheaper to just full send it than produce a full run at a given quality with a low rejection rate.
The "old" way of making a black fan is you just QC check them, send the good ones to Noctua, send the crappy ones to someone who DGAF because they're putting them some sort of industrial appliance that needs airflow through the box.
Everyone "wins" this way because Noctua gets their fan to spec cheaper and the people building plasma cutters or control units for chemical washers or ATMs get a fan that's "fundamentally good" if sloppily executed and the manufacturer gets less waste. Ain't no different than how the pork belly that doesn't become your bacon becomes dog food and die lubricant.
I suspect this is where a lot of the "X compatible" power tool stuff on Amazon comes from. That and/or the repurposing of "worn out" dies.