Chinchillas won't always breed right away, this is a well known fact. When you breed in runs, sometimes a male will have four to six females and you could have other kits growing out when you put the run together that you may also want in the run in the future. Say it takes a male one month to get the job done with three females and three months for the others. After three months, your grower is of age and did well at show and you'd like to place her with him. You have three females already visibly pregnant and the other three not yet showing signs. Maybe after looking at the animals and re-evaluating them, you decide that one of the females not showing signs isn't as good of a pairing for the male as you thought she was compared to your grower. If you don't have a different male to put that female with, breeders are not going to hang onto her for four months to see if she litters.
For large breeders, they check the cards to see when the female last littered and what males she was placed with. If she hasn't produced in two years, she gets pulled and sold or pelted. This is how I've gotten a few very nice standards and their babies from lines unrelated to mine. As Tara said, it's a two-fer and can be very nice when it happens depending on how the kits turn out.