the insulation takes the form of the strands

Good day!
I have been thinking for a while now,but depending on the lay length of stranded cable ( not in the case of a stranded conductor) the lower the lay length the less ,its strand is distinguishable after its jacketing. But what makes this defect happen? is it the shrinkage of the jacket due to heat that takes the exact form of the strand? or maybe is it the pulling tension that makes it take the strands form?.what do you think?.

mostly tension, you'd better to work with a wire drawing machine with quality sensor .