Several pages on the NGC website underneath "Design" have a reference to "online casino gambling". Here is a screenshot of one such example:
I've seen a few others in the past week that weren't related to gambling but was definitely off the wall.
That price guide seems pretty poorly attended to- sometimes it will say $20 for AU and then $5.25 for MS63
Someone over at NGC must of been playing on a casino website during thier break and caught some kind of virus bug... Now I know why they take soooo long to get my coins slabbed