Question, I'm confused

[leprechaun]

In mens league slow pitch softball (ASA rules). Runner on first, 1 out. Batter hits a grounder to first. Runner on first does not leave the base. First baseman tags runner on first and then steps on the bag. The umpire ruled batter was out, but runner on first was not. I found a rule about two runners on the same base stating that the last to arrive is out, my question is is there a difference when the runner must advance, or does the runner not have to advance. Just confused is all. P.S. We still won the game so it really doesn't matter, just wondering.