"So they went with haste and found Mary and Joseph, and the child lying in the manger. When they saw this, they made known what had been told them about this child; and all who heard it were amazed at what the shepherds told them. But Mary treasured all these words and pondered them in her heart."
Matthew's story of the immediate aftermath of the birth of Jesus involved wealthy, educated magi from the East; Luke's story is of humble shepherds inspired by angels on high. Together they emphasize that the birth of the Christ spirit is of great spiritual importance for all people.
As for Mary, she doesn't trust her mortal mind to understand or explain what the birth might mean. As with her own visitation from an angel at the time of the Annunciation (Luke 1:26), she accepts and submits, trusting in the spiritual guidance of her heart to make sense of things as she looks back in peace.