Oxidation gets pretty deep into the gelcoat, and sometimes you think it's gone but it comes back. We used to have a plane like that at my old work - after 8 hours of compounding, it looked perfect. 3 weeks later - chalky again.
Sometimes you can cut deeper or wetsand and get all of the oxidation out, sometimes the only long-term solution is to repaint. If the SA3 side looks pretty good, then the competitors product probably didn't cut deep enough.
Klasse is an interesting choice - I always preached the oil glaze and Collinite gospel when I worked on gelcoat, but I'd be interested in hearing how it looks/lasts.