Also in a pool with a lower FC/CYA ratio the kill times against pathogens is lower, but in practice it's still pretty high and for most pathogens the kill time is faster than reproduction time. So there is no uncontrolled bacterial growth. For residential pools, this is probably fine, but for commercial/public pools a faster kill time is desired to prevent person-to-person transmission of disease. Algae is harder to kill so that's why in practice for residential pools it's the main reason for the minimum FC level relative to CYA.