What an excellent question! One theory is that babies need lots of cream in the evening before their longer sleep. With the milk volume being lower at this time, the baby gets a much higher percentage of cream. They eat very frequently to eventually get the volume they want.
Doesn't my nany's brain drive the composition? Yes. Lots of cream only comes from a lower volume in the breast at the time the baby feeds. In many cultures mothers go to sleep at this time, so it doesn't really matter how often the baby eats, as mom is dozing and getting much more rest than most American women.
Babies stay the same, lifestyles change.