Why is Ruth's child in the line of Boaz and not Mahlon?

It seems to me that according to this law the first-born child of Boaz and Ruth should have been recorded in the line of Mahlon: >If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her. And it shall be, that the firstborn which she beareth shall succeed in the name of his brother which is dead, that his name be not put out of Israel. *Deuteronomy 25:6 KJV* Boaz clearly mentions this when he purchased the land: >Then said Boaz, What day thou buyest the field of the hand of Naomi, thou must buy it also of Ruth the Moabitess, the wife of the dead, to raise up the name of the dead upon his inheritance. *Ruth 4:5 KJV* mar mine own inheritance - is why the other relative was unwilling to redeem the property. Yet, Jesus, David, Solomon were all recorded as belonging to the lineage of Boaz, not Mahlon. Questions: 1. Why was it recorded like that? 2. Is this the norm or the exception? 3. Is there any other recorded incident in the Scriptures where this was done and the lineage was accorded to the deceased person?