why if 10 tevet falls on Shabbos we still fast on Shabbos? but other we shift on other day?

Shalom uvracha, According to the shulchan aruch (550 3) if the tenth of Tevet fall on shabbos we fast on Sunday. The abudraham held that if the tenth of tevet falls on shabbos we fast on shabbos and he learned this from a passuk in sefer Yechezkel בעצם היום הזה (see beit yosef 55). However as we mentioned the hallacha is that we fast on Sunday. All the best