As I understood it, the drive works as follows: (note that some of this is speculation)
1) the ship is directed to teleport to a specific spot (an immensly large improbability but finite that is likely denoted short hand by infinity minus one against).
2) in order to reach that improbability, the drive must generate each lower finite improbability (1:1 to 2:1 to 1,000,000:1 etc.)
3) at a given improbability the "normal" outcome of doing something will be the outcome with the probability the drive says should happen. For example: at 1:1 things are actually normal; at 2:1 a coins could land on tails or a dice on an even number; at 10,000:1 the coin would land on its side; at 10^1,000,000,000,000,000,000 somehow the coin lands on fish, monkeys type hamlet, and an ocean forms that stays still while the land has waves. The actual events are chosen for humorous effect.
4) the hitchhikers arriving was at a specific improbability that was massive (infinity minus one) but less than the intended teleport. Teleporting to a random location in the universe is more likely than teleporting to a specific one so it teleports to all locations randomly.
5) once the improbability to go to that specific place is reached, they teleport to that location (quite easy iff that is the normal thing for the ship to do now) and start bringing the drive down.
6) as the ship returns to the true 1:1 "normal" (whatever "that" is anyways), most things that have transformed or materialized turn back or disappear respectively. Apparently these apparitions are still inherently unlikely so change back as the drive goes back. Some things are more improbable to transform back than remain as they are so are unintentionally still in existance when "normal" is reached. This includes (for instance) the redecorated ship, the passengers remaining on board, and the eggs and confetti remaining to land on the (nearly) deserted planet.