Ben 10 S4 Episode 2 REACTION

As you mentioned, the watch seems to have its own mind. The watch has a lot of alien DNA, but the default is just 10 aliens. When the watch deems it necessary, it grants Ben access to new aliens, and he keeps them after unlocking them. Cannonbolt, Wildvine, & Ditto were all given to Ben by the watch. Upchuck on the other hand was already in the watch but was not given to Ben until Grampa's Ex unlocked him. (Showing that there are more aliens in the watch) Benvicktor, Benmummy, and Benwolf were all acquired by extracting DNA.