Most comedians use the same routine. They develop a routine, usually 30-45 minutes long, and use it every night. Sure, there are slight variations, but mainly the same.
In these smaller segments, a comedian has to decide what he can do in 3-5 minutes and get the best result.
Sure, I'm sure most people would love to do diff bits each time, but when you have that limited amount of time, you need to deliver the best impression possible. If you've ever noticed, stand up comedy is rarely a bunch of unrelated jokes put together.
Each joke, independant from the other.
More likely, comedy is kind of a story, where a lot of diff jokes depend on other moments to really get maximum effect.
So comedians have to be able to choose which segment can be taken out and be independant from the rest but still be pretty effective.
Well, that's just my opinion anyway. I find that I (as someone listening) have to be warmed up to the comedian and then I kind of "get" what he's about and I'm ready to laugh with him.
Oh, and day old jokes. I highly doubt, this is the first time (or second) time he's used this material. Comedians hone their material before really unveiling it for the big audiences. I think Jay Leno still goes to Vegas each Sunday to try out a ton of jokes for the next week's worth of shows.