Ask Him.
What I do know is that the title expresses a truth that is in the New Testament: Jesus is God and Mary is His mother, and she intercedes with Him on our behalf. For example: The marriage feast at Cana.
Nowhere in Scripture does Mary intercede with Jesus for us Arthur.
Nowhere in Scripture does God - or anyone else - call Mary the Mother of God.
Why add to His Word? It is sufficient as He inspired it.
Best