I believe it occurs most commonly with dual monitor setups

In case you are still confused, let me explain - Nvidia knows how to advertise their products and their gimmicks better, let’s be real. This means more people, especially new customers will be convinced by the features they advertise with their cards, and that’s how they win their customerbase. Another factor is that AMD hasn’t had a very good track record with their previous cards and software, many people still hold their skepticism and posts about driver issues here and there don’t help it either. I could also mention the availability, there are parts in world where the availability for Nvidia cards is better, so in these cases the customer has no other choice but to go with Nvidia.

The sales numbers are kinda irrelevant, when the market share gap is so big. It’s kinda common sense at this point, that the Nvidia counterpart would sell better and to that there’s more factors than whichever card actually has more features and better price to performance.