Top heavy fractions with same degree

Quick question, all fractions where the degree of the numerator and denominator are the same are considered top-heavy. However, take into account x² as the numerator and x² - 1 as the denominator for example. They both have the same degree, but if you substitute any positive integer for x, you'll get a proper fraction, i.e., one that isn't top-heavy, therefore why is it still considered so?