When you really think about it

In Korea when a baby is born they are counted as one year old. Even though a baby is only in the womb for 9 months. So every child that is born over there is automatically 1 yr old

It is because some babies are not born in nine months, some could be late, or premature.

Also, different countries have different laws about when a developing baby is legally a human being. Like sometimes its 4 months, 5 months etc.

Or because its called a BIRTHday, not a CONCEIVED day. You aren't born yet when you are chillin' in the womb.

I was born on the due date