Top class horses get beaten by inferior horses all the time for very good reasons other than lack of ability or luck in the running on the day.

No point backing any horse unless it is ready to win. In other words the conditions suits the horse and is the optimum period[s] of it's campaign: Could be a 1st up or 2nd up winner or a 4th to 8th up winner, the distance must suit as there are often some class stayers prepping in sprints for longer races etc. and a heap of other condition details to look at.

The old saying 'you can't lose by backing class' is absolute myth. When to back it is more to the point and that is no easy question to answer.