The explanation is even simpler than that.
When Mary was pregnant with Jesus, His Entire Being resided in her womb — not just His humanity but also His divinity.
Christ received His humanity through Mary (all humans have mommies) but His divinity was ‘From the Beginning....’.
The two natures fully resided within the Fetus Mary carried, and she can therefore rightfully be assigned the title ‘Mother of God’ without herself being divine.
The Divinity part is still a bit of a mystery since we know that the supposed Divinity of God withing the flesh of Jesus didn't know the day nor hour when Jesus would return to set up his Kingdom...However, that may be as simple as God hadn't made up his mind specifically on what day he would return...
But since we know that Mary was not the mother of Jesus' divinity insofar that the divinity was provided 100% by God, it's akin somewhat to a female dog nursing and raising a kitten...Does that make the dog the kitten's mother???
Or suppose with science that it now is, if a pig fetus was planted inside a woman and the pig was born, is the female human the mother of the pig??? Or just the bearer???