On MicroSD Problems « bunnie's blog
"Effectively, Kingston is just a channel trader and is probably seen by SanDisk/Toshiba as a demand buffer for their production output. I also wouldn’t be surprised if SanDisk/Toshiba was selling Kingston “A-” grade parts, i.e., parts with slightly more defective sectors, but otherwise perfectly serviceable. As a result, Kingston plays a significant and important role in stabilizing microSD card prices and improving fab margins, but at some risk to their own brand image."
Apparently Kensington releases SD cards produced on "Ghost Shifts", with sub-standard materials.