[csense]

I think it's selection bias.

If there are ~100 fans who make trailers for every major movie that comes out, a lot of them will probably be garbage, but this article isn't comparing the official trailer to a randomly picked fan trailer; you're comparing it to the best or most viral fan trailer of the 100, the one that gained enough traction to be visible.

Also, since (in the scenario in this article) the official trailer comes out first, and the fan trailer copied footage and such from the official trailer, it's not too hard to imagine that bootstrapping from the work done on the official trailer helps the fan trailer overcome the handicap of having resources several orders of magnitude smaller (probably consisting entirely of a couple fans' free time and, if they're lucky, maybe commercial movie editing software that one of them had bought for something else.)