Is it possible the oil life % was reset without you knowing it, which restrted the counter at 100% ... say after 4000 miles, or something like that.
In all my Chevy's (including my car), to reset the oil life %, you turn the key over, but do not engage the engine. You then press the gas peddal 3 times in a row to the floor and stop. This resets the oil %. You then start the vehicle normal.
It's possible someone could have pushed the peddal 3 times in a row before actually turning the key over. Older (much older) vehicles, you use to have to pump the gas peddal first to pump the gas to the carberator. Sometimes (and I'm old enough to remember this), even I have a tendency to pump first (when it's completely not necessary). That's just part of my eary teen years I guess.
Anyway, I wonder if the oil % got reset without you knowing.