They feel the browning falls under the "ST" qualifier, and the card is otherwise NM. Kind of useless, as if they graded it without the qualifier they would have to give it a very low grade. Qualifiers were an attempt to address specific defects that might bother some more than others (like centering). It's a way of giving the buyer information about the structure combined with info about a specific defect.
Having said that, it's inappropriate, because this browning is NOT a stain, anymore than a whiteback with a tanned border is. They don't qualify or downgrade for that tanning at all, to the chagrin of many.
