Well, when Ben first got the omnitrix, it gave him a set of aliens. When he put the omnitrix back on in Alien Force, it recalibrated because he got older, and it gave him a new set of aliens. When he put on the Ultimatrix, it didn't recalibrate since it was the first time Ben (and Albedo, which doesn't really matter since he had Ben's DNA)had ever put it on. Since he was new to the Ultimatrix, it then scanned his DNA and found all of the DNA of all of the aliens he had ever turned into, and it gave them to him. They are in the intro because he can turn into them again.