Also the price usually stays the same, so there is more material per diaper, so less diapers. But often times as they get older they pee less frequently (bigger pees), as there body gets ready for potty training
My daughter almost a year old and she pees like crazy. Probably changing her almost 10 times a day between pee and poop. I bought her thicker diapers to help absorb as much as it can before a change is needed. Like I use pampers but I’ve noticed pampers baby dry are thinner compared to pampers swaddlers. Diapers drive me absolutely insane
Normally it’s only a few less so he probably just accidentally bought the way smaller size 4. But they do go down when they get bigger because they either are peeing less or they get closer to potty training