Please Experts of meritnation give the answer fast . Ross??birthday falls on the same day as mine. Why is the inverted comma oitside the word not in the middle of last two letters

When any name or noun word ends with 's' at that time inverted comma is given outside the word .
For ex - sparrow - sparrow's
(do not end with s )

princess - princess'
( ends with s . So outside the word )