Seems to me that there may be two reasons as to why people will say they hit fewer trees.
1. You don't get mad over hitting a tree. You are having fun and if you hit a tree, you just accept it and go on. At the end, you probably don't remember hitting them because of this.
2. You focus more on executing the throw. You know the trees are out there, you just can't see them, so you make better throws that don't go off line. In the end, you really do hit less trees. This could be why people say they score better at night.
When I played glow rounds, I seemed to hit about the same amount of trees, but I just didn't get mad at myself because I was having more fun than usual and didn't care.