Tuan Jim Isn't this what every comic does? They have a bit and because it's live it comes out a little differently each time, maybe they are refining it a little?
Dread Pirate Roberts They also have to listen to their own act 60 times a month. Changing it up a bit allows them to not get into the repition of it. They change words, adjust timing, etc. Helps keep it funny for returning patrons, and also helps keep the comic sane. Being a comic means you have to be able to smile and laugh at stuff too. Rarely is there a comic that won't break character once or twice a show to smile with the audience a bit.
Toenails I tried to look for a video with Steven Wright smiling with the audience, but I think the internet gave my computer a virus instead.
RomancingTrain It's from the DVD, I imagine it's for completionist fans that must see every version of the joke.