Captain Beatty is using the example of Clarisse to show that regardless of the attempts at homogenizing the society a person’s heredity and the environment can still produce someone who thinks and acts differently.
“Here or there, that's bound to occur. Clarisse McClellan? We've a record on her family. We've watched them carefully. Heredity and environment are funny things. You can't rid yourselves of all the odd ducks in just a few years. The home environment can undo a lot you try to do at school. That's why we've lowered the kindergarten age year after year until now we're almost snatching them from the cradle."
Even though not meant to be, Beatty’s statement is one of optimism. People can be different; people can change regardless of their environment. The characters of Clarisse and ultimately Montag give this redemptive view of mankind.