Each Bob clone has a unique personality that is slightly different from the Bob that cloned him.
The first example is cited in We Are Legion (We Are Bob): of the first cohort of clones that Bob creates, Mario is described as being somewhat antisocial. Unlike his cohort – who stay in their home system or choose nearby systems to explore – Mario decides to explore a distant system requiring a lengthy transit, guaranteeing that he will be incommunicado for decades.
With each successive generation of Bobs, these personality differences become more pronounced. By the 20th generation, the clones are quite different from Bob 2.0.
One example is Lenny, who in Heaven’s River is the spokesperson of the faction of Bobs called Starfleet. He tries to force his will on the rest of the Bobs, which the early-generation clones find abhorrent.
During a conversation with Bob, Hugh explains that the results of personality tests pre and post cloning indicate that if a Bob is shut down and copied to a new matrix, and only one of them is reactivated, there is no drift. However, if the other matrix is reactivated, it always shows replicative drift (even if the physical matrix that's turned on last was the original hardware). To this end, because there was no overlap in existence, Bob - despite being a replicant - is actually a direct continuation of Original Bob .