It has to do with the word we're adding s to. Normally, 's is used. But, if the word ends with an s, then the apostrophe comes after - like s'.
Jenna's jacket , Jenna doesn't end with an s = 's
Chris' jacket , Chris ends with an s = s'
It will be the same with other nouns, for example: The boy's ball (one boy) and The boys' ball (several boys)
And to add, that's often how you know whether or not the possessive word is plural. "The boy's" is singular because the apostrophe denotes the end of the root word. Whereas "The boys' " is plural (as an something belong to a group of boys) because the apostrophe denotes the end of the root word, which is "boys".