This is because the original charts produced in 1977 were based on samples of middle-class white American babies on high-protein bottle-fed diets in Ohio.
ðɪs ɪz bɪkˈʌz ðɪ ɚɹˈɪdʒɪnəl tʃˈɑːɹts pɹədˈuːst ɪn nˈaɪntiːnhˈʌndɹɪd sˈɛvənti sˈɛvən wɜː bˈeɪst ˌɔn sˈæmpəlz ʌv mˈɪdəlklˈæs wˈaɪt ɐmˈɛɹɪkən bˈeɪbiz ˌɔn hˈaɪpɹˈoʊtiːn bˈɑːɾəlfˈɛd dˈaɪəts ɪn oʊhˈaɪoʊ.