Even though the positions of the three protons on the original phosphoric acid molecule are equivalent, the successive Ka values differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged.
ˈiːvən ðˌoʊ ðə pəzˈɪʃənz ʌvðə θɹˈiː pɹˈoʊtɑːnz ɔnðɪ ɚɹˈɪdʒɪnəl fəsfˈɔːɹɪk ˈæsɪd mˈɑːlɪkjˌuːl ɑːɹ ɪkwˈɪvələnt, ðə səksˈɛsɪv kˈɑː vˈæljuːz dˈɪfɚ sˈɪns ɪɾ ɪz ˌɛnɚdʒˈɛɾɪkli lˈɛs fˈeɪvɚɹəbəl tə lˈuːz ɐ pɹˈoʊtɑːn ɪf ðə kˈɑːndʒuːɡˌeɪt bˈeɪs ɪz mˈoːɹ nˈɛɡətˌɪvli tʃˈɑːɹdʒd.