One of the Gospels gives Mary's descent, the other one, Joseph's, so that both legally and physically Jesus would be born of the House of David.

To realize what happened here is actually to realize the incredible ingenuity of God. You see, there was a curse on David's line through Coniah that no seed (children of his by blood) would prosper or be able to reign. The only way this could be bypassed is for Jesus to have Joseph as his legal father to inherit the promise but not be by blood (aka, born of a virgin). The Scofield explains this really well in the note for Matthew 1:11 ======================================================================================================== 1:11 Jechonias. This man is called Coniah in Jer. 22:24-30 where a curse is pronounced upon this former king of Judah. There it is predicted that none of his seed should prosper sitting on David's throne. Had our Lord been the natural son of Joseph, who was descended from Jechonias, He could never reign in power and righteousness because of the curse. But Christ came through Mary's line, not Joseph's. As the adopted son of Joseph, the curse upon Coniah's seed did not affect Him. ======================================================================================================== Also, I would like to add that the possibility exists that Mary came from David's line as well, but not from Coniah's, thus avoiding the curse, so Jesus still came from David's line but not Coniah's, upon which was the curse. This would explain why the genealogies in Mt. 1:1 and Lk. 3:23 differ. The Scofield note on Luke 3:23 gives a good explanation of this: ======================================================================================================== 3:23 THE GENEALOGIES OF JESUS The genealogies of our Lord recorded in Mt. 1:1-17 and Lk. 3:23-38 have their similarities and their differences. Though the Lucan genealogy goes back to Adam and that of Matthew goes only to Abraham, they are both in absolute agreement in the generations between Abraham and David. It is with the Son of David that the great difference begins, for Luke traces our Lord's ancestry from David through Nathan, whereas Matthew uses the royal line through Solomon. It is true that the names Shealtiel, Zerubbabel, and possibly Matthat (Matthan in Matthew) appear subsequently in both, but otherwise the lists are entirely different. Indeed in one, Jacob is spoken of as Joseph's father, whereas in the other, Heli is presumably so presented. Two views have been maintained by equally godly and learned scholars. Some believe both genealogies are of Joseph, but that the one in Matthew gives the legal descendants of David to establish our Lord's claim to the Davidic throne, while Luke gives the particular line to which Joseph actually belonged. The second list, then, is spoken of as the collateral line and is eligible for royal duty when the legal line is incapacitated or becomes extinct. A far simpler solution, and in all probability the true one, is that since every individual has two genealogies--one through his father and another through his mother--so Matthew presents Joseph's genealogy (the Lord's foster or legal father, not his actual father), whereas Luke presents Mary's genealogy. This view is supported by linguistic and historical evidence and is held by many students of the Bible. In addition, appeal may be made to Num. 27:1-11 and 36:1-12 to give Scriptural precedent for the substitution of Joseph's name in Lk. 3:23. At the same time it avoids the judgement spoken of in Jer. 22:28-30 (see Mt. 1:11, note). ========================================================================================================

Mary was also of the house of David.

That question, for most of us, is one which can be answered only by those who truly know the scriptures, and even then would not likely be in absolute agreement with one another. Try to remember, though, that GOD does not make any promise which He will not keep.