Guessing it has to do with pluralization.

further bit: accidentally tried to add a card without a name, and got warned, "NAME: must be unique-- A card named 'S' already exists"

  --John Abbe.....Mon Feb 28 22:25:40 -0800 2011

the key for "s" is "". unfortunately this is not as simple as you might expect -- involves tweaking the key generation algorithm. though I though by now we were fully disallowing blank keys.

  --Ethan McCutchen.....Mon Mar 07 16:49:33 -0800 2011

this works now, though some "s" cards may need to be resaved to fix the weird key.

  --Ethan McCutchen.....Fri May 06 22:20:18 -0700 2011