That absolutely makes sense, daycare centres make their profit by volume not like us. Also as you said their ratio changes once they turn to be toddlers or preschoolers. Their groups are larger. Absolutely not a comparison with our rations.

I might be wrong but I think it's a practice that is common in day care centers. With the ratio changes, the infant spaces in a center are hard to find but the older age groups are rarely at capacity. By offering a sibling discount, they can retain the older children.
Thank you Suzie_Homemaker.