Because Prolog searches for clauses from top to bottom, the order in which they are written makes a difference. If a predicate has several solutions, their order of appearence is determined by the order of the clauses.
If the conditions in alternative clauses are not mutually exclusive, then backtracking will produce them both:
has_fitness(Skier, poor) :-
max_pressups(Skier, P),
P < 15.
has_fitness(Skier, good) :-
max_pressups(Skier, P),
P >= 10.
max_pressups(eddie,12).