It's because women vary in the number of areolar glands that they have, and there is a correlation between the ease with which babies start to suckle and the number of areolar glands she has.
女性によっては 持っている乳輪腺の数は異なり どの赤ん坊がやすやすと おっぱいを飲み始めるか というのと 女性の乳腺の数には