It is because Lent in the Roman rite originally began with the Vespers of the first Sunday (as it still does in the Ambrosian rite).  Additional days were added, quite early, as fasting never applied to the Sundays, to make a total of 40 fast days.

This means, as you note, the Lenten hymns such as Audi, benigne Conditor, Ex more docti & O sol salutis begin on Sunday and for Ash Wednesday and the next three days the hymns are as in Septuagesima.  However, there are the ferial preces at the Hours when the office is of the feria and the extended kneeling at Mass.