They're RNG but ships and sparrows share the same vehicle drop pool. As you get more points the more likely you are to get one of the vehicles (at 1000 points you're guaranteed one), once you get one the drop rate goes back to normal then you build up more points again. The pools are on a knockout list so you were getting things you hadn't already had and RNG wasn't in your favour. In future Trial weekends you'll be able to keep working through the list.