No idea but have wondered this too. I do know that when I first wash the come out nearly dry so don't even obsorb the washing machine water to much but well washed nappies take much longer to dry and come out quite damp. Hope some comes along with an answer.
The other thing is that anything with cotton or hemp in usually the fibre is coated with 'sizing' which is an oily substance to smooth the (rough) fibres so they go through the machinery to make the thread/knit or weave the fabric without snagging. Obviously when you buy new cotton it feels silky smooth - but you have to wash the sizing out and that also increases the absorbency (think new towels how they repel water)