I *think* it's because there's 4 kids per carer for the older room, but 5 kids per carer in the baby and toddler rooms.
It is definitely due to ratios but I think the above are not right - the ratio is more kids per carer the older they are, and for over 36 months is something like 1:10 whereas under 3 is 1:5 and under 2 is 1:4. Mixed age groups the ratio is proportional to number of children in each age. As they have high number of children per carer in older rooms (3-5), then is difficult for them to have one go off and to assist with toileting / change nappies etc. (Centers may have more staff per room than the ratio demands)