In short, "pre-approving" allows the guest to book if they want. However, they do not have to, so someone else can come along, also get a pre approval and then book first. Each guest has 24 hours to book after you pre-approve them. The first time this happened to me I was a bit confused as well, and I felt sort of sorry for the first person, like they really had the "right" to book. Now, I am totally comfortable with the system. I do let them know, however, that someone else is inquiring about the dates, just as a courtesy.