That is not actually true. There have been posters after the first board that already received the....We are sorry, but unfortunately we are unable to... letter.
Some, even many will be reboarded for a simple reason that HQ has decided that they will probably receive a scholarship, but unsure of which type. IE you may be on the cusp between a type 2 and a 7 or a 7 and nothing, thus they want to see a larger pool. However, it is important to realize that only @16-18% of all applicants receive a scholarship which means many will not be reboarded because by the second board they will have a very strong picture from historical data what would the bottom score needs to be at on the WCS to get a type 7, hence increasing the likelihood that being reboarded is not going to occur.
Sorry to burst the bubble, but I don't want posters and lurkers to believe it is a given of reboarding.