Was thinking something similar. The natural oil/sebum probably spreads or "travels" down each hair shaft w/the help of time and gravity.
Straight or slightly wavy hair tends to get oilier more quickly after a wash, maybe because it's easier for the sebum to move down a straighter shaft that has very little bend to it. I suppose angle of hair would matter too. If it tends to grow "up" rather than down (I suppose) it could make it hard or impossible for the spread of sebum (unless the hair is manipulated into a downward position). Enter natural oils to compensate.