Selection officers do not consider the preference level of their course when deciding whether or not to make you an offer. This means that when they are deciding if they will you offer you a place in their course, they do not take into consideration how high or low their course is on your list of preferences. The order of your preferences will not disadvantage your application in any way.
In this example the applicant will be unaware that their second preference chose not to make them an offer or that their third preference would have made them an offer. The only offer they will receive is the one made by the highest preference – in this case their first preference.
Trying to second-guess the system by putting your dream course second and the course you're sure you'll get an offer for first will most likely result in you getting an offer for your first preference and you'll never know if your dream course would have made you an offer or not.