Thank you for this! Yours is the most astute comment I've seen here. As someone who's worked in flash memory manufacturing, I know how much variation is inherent in the process. It gets evened out somewhat by binning, but there's inevitable variation in dielectric thickness, aspect ratios, corner profiles, dopant diffusion, grain structure of polysilicon, and on and on, even discounting the density of particulate defects and watermarks.

Even if you don't know that, basic stats should tell you that testing a single unit from each manufacturer is going to be meaningless at best, misleading at worst.