Showing singular possession for words ending in "s"

This past weekend, my mother, sister-in-law and I discussed the proper use an apostrophe when indicating singular possession for words ending in "s". The subject came up when I said the St. Paul Pioneer Press used "Phelps'" in one of their Olympic headlines. I argued this was incorrect as it indicated possession to the Phelps family instead of the singular Mark Phelps.

Moreover, my argument was, even if a word ends in "s" we should use an apostrophe "s" (i.e. Phelps's) when the noun is meant to be singular. My sister-in-law was wishy washy, but decided I was wrong and that adding an apostrophe at the end was sufficient to show singular possession. She was agreeing with the Pioneer Press. My mother was indifferent.

Turns out we are both right. Which is not surprising with English. The general idea is you should add an apostrophe "s" (i.e. Phelps's) when showing singular possession, in most cases. Oddly biblical terms like Jesus you should use "s" apostrophe (i.e. Jesus').

I've love to hear an argument saying the Pioneer Press was right.