Pat has talked before, albeit briefly, about what's on his cards.
For example, letter distributions are not on the cards, those are directed to him offstage via a small TV monitor. At one time, they used to be directed to him from a person within Pat's sight (sometimes producer Nancy Jones) that holds up the number of letters in a puzzle after the letter is called. The former live person and current monitor are positioned very near or under the camera that focuses in on Pat most of the show.
As others have said, the bonus round grand prize info is directed to Pat from offstage. The puzzle categories are on his cards, but things like what wedge the $10,000 is under during the mystery round aren't. The bonus puzzle IS on his card, and I'd bet he does everything he can to keep it away from the player's sight. It's all nothing more than general production inclusions to help speed things up, not because of something minuscule like Sajak not wanting to be surprised... -_-
/If you believe that it is though, give me your address and I'll send you my pamphlets about Florida swampland, wedding rings formerly owned by Larry King, and houses for sale at the North Pole.