Why do Polish surnames change with the person's gender?

Polish surnames are adjectival, meaning that they behave like adjectives in a sentence. Because of the gender-agreement between nouns and adjectives, an adjective will sometimes change the final vowel depending on the number and gender: duży chłopiec 'big boy' v. duża dziewczynka 'big girl'. The exact same thing happens with surnames: Maria Młoda v. Jan Młody. Even if they change vowels, they are still the same surname.