I’m not sure what you mean by SSD grade, Grade A to D chips aren’t strictly about binning but also traceability/fraud.
One hardware guy mentioned internal defects can cause differences is the amount of reserve sectors that a final product ends up with. That’s exactly the kind of arbitrary cutoff that lets companies charge different prices for the same part.
Lower-grade flash with higher initial defect rates is what gets used in USB flash drives and SD cards, and some bargain-bin SSDs with lower usable capacities (ie. 960GB rather than 1TB).
The stuff used in a WD Black or WD Blue branded consumer SSD is not a different quality grade from the stuff used in any other mainstream consumer SSD, Apple's included.
One hardware guy mentioned internal defects can cause differences is the amount of reserve sectors that a final product ends up with. That’s exactly the kind of arbitrary cutoff that lets companies charge different prices for the same part.