Some tactical positions have moves, which are good, but are not deemed to be the 'best'. When this occurs, you will receive a 'try again' message, and you'll be given another chance to find the 'best' move.

The reason the problem set includes positions with these alternative winning moves is because it is important to provide training in as many real game situations as possible. Real games often have positions where you need to choose between multiple good moves, and so it is important that training includes such positions. Positions where two moves are given the same computer evaluation will not be used, so one should always be better than all others (at least from a computer evaluation perspective).