Consider an ennemy Galton-Watson process where each node has 20 offsprings on average. If you kill 90% of the nodes, is that a success? No really: the remaining nodes are still a rather impressive number. How to destroy them, then? The solution is to repeat the destructive process a second time. If the second attack kills 90% of the remaining nodes, then you will be left with just a few sparse isolated surviving individuals who can then be killed one by one as they come out of their hiding places.
Apply this to flea infestation: I am lauching my second chemical weapons attack today, with stronger chemicals that leave a residue on floor and furniture, so that any egg that might still hatch in the near future would then be killed as soon as it is born.