i think Lailai is meaning:

il = average point

a average point is how many points it can be off. most commonly it will be the average, but there is a possibility that there can be one average point away from it. or even two. you are luckier if you have 3 points off the average. but a child under the average range does never give you luck.

a average range is as big as 3 average points. or even you can excpect a 4-point-off or even 5-point-off which produces a very special child.

it can be 1 average point long, 2 average points long, 3 average points long, or even 4 average point or even 5 average points long. but never 2- or 3-, because the most child ALWAYS is the middle. so, every parent has 5 il. no exceptions.