It's not uncommon for cats to still have fleas after deworming. This is because the flea life cycle is very complex, consisting of four stages: egg, larva, pupa, and adult. Of these, only the adult fleas parasitize the host, while the eggs, larvae, and pupae mostly hide in the environment.
When you deworm your cat, if you only kill the adult fleas and don't deal with the eggs, larvae, and pupae, these remaining fleas will quickly multiply again, causing your cat to become infected again. Furthermore, flea eggs can survive in the environment for months, meaning that even if your cat has been dewormed, they may still hatch into new fleas at some point in the future.
