When I did the ATPL Theory I was often reminded that when presented with a selection of possible answers,pick the one that is "most" correct. Since some texts show approx 39,000 ft and your choices are 38,000-40,000 ft or 40,000 ft then by excluding those that are obviously outside of this range you have your answer for exam purposes! A scientist may give a better explanation ...? The differences must not detract from the question just try to accept that this is fairly typical for JAA questions.