Why would God have granted Mary such a gift? When the eternal Word took on flesh (see Jn 1:14), he took his flesh from her. God wanted his sinless Son, Jesus, to receive his human nature from a sinless mother. And it was most fitting for Jesus to be reared by a woman without sin.
This is also a matter of common Hebrew idiom. In Romans 3:10-12, St. Paul quotes Psalm 14:3, which reads: “All have gone astray; / all alike are perverse. / Not one does what is right, / not even one” (see also Ps 53:2-4). Yet the very next psalm refers to those who walk “without blame” (Ps 15:2). Obviously, then, the lament in Psalm 14:2-3 is emotional and exaggerated language, not intended as a literal utterance. Since St. Paul is referring back to these sorts of passages, the interpretation of his words should take them into account, too.