that's just cause Audi's recent marketing campaigns have been so successful
as jeremy clarkson once said: BMW's used to drive great, but the only people who bought them were cocks...meanwhile audi was quietly making some of the best cars on the road, and you could own one without feeling like an asshole...now it's the other way around
so both companies make great cars, it just depends on what you think about the brand and its imaging, it'll keep fluctuating like that