Note the difference in what God said in the two Gospels...
17 And behold! A voice out of the heaven saying, This is My Son, the Beloved, in whom I have been delighting.
Mark 1:10-11 And going up from the water, immediately He saw the heavens being torn, and the Spirit coming down as a dove upon Him.
11 And there was a voice out of the heavens, You are My Son, the Beloved, in whom I have been delighting.
These two passages are clearly about the same event but what God said was different as Jesus came out of the water. It appears something must be in error. But instead of trying to explain away the contridiction, lets take the position that God wrote the bible with messages for us to learn and that his word is without error. If this is so, we are compelled to dig deeper to see why God wrote it this way. There is always a reason.
We assume the gospel writers were writing about history and they were, but God does sometimes change the way a story is told to fit the teaching he has for us. I don’t think anyone can speak for God and say what his rationale is for this, but the words fit the context in a way only God could have put together. It is more important to God that we get the message than it is for us to evaluate how it is written.
Both these verses are exactly correct when considered in the context in which they are written. These are not two interpretations, but are two perfectly correct statements for the message being communicated in each Gospel.
In Matthew 3:17 John (3:2-3) was preaching and telling people "Repent, for the Kingdom of Heaven is near. This is he...." John is telling people who Jesus is as if point to him saying " This man is the one...." Also in 3:17 the Spirit of God was not only decending on Him, but also "lighting on Him"...a spotlight for humanity to see?
So in this Gospel it is appropriate for God to be making the statement from heaven in the same way as John. It was directed to people observing the event..."Look, this is my Son, the Beloved...". The context of the message supports what God is saying in it.
Mark chapter 1:1-39 is about the authority of Jesus. You can see this in many other verses in the same chapter. Verses - (7) after me will come one more powerful than I; (11) you are My Son; (17) come, follow Me; (22) because he taught them as one who had authority..; (24) I know who you are...the Holy One of God; (27) a new teacher, and with authority. He even gives orders to evil spirits, and they obey him; (34) he would not let the demons speak, because they knew who he was.; (38) Jesus...Let us go somewhere else....so I can preach there also. That is why I have come.
So in Mark 1:11 where God writes "You are my Son, the Beloved...", he is not speaking to observers. Instead, God is bestowing His authority on Jesus. It is God telling Jesus - you are my Son and by virtue of that, you have my authority. The meaning of the verse fits the context of the writing perfectly.
Nobody really knows exactly what fully transpired at this event, but we do know what God tells us about it in two different gospels. Each is correct and in each one, God is making a point using the words necessary to do so. I believe God wants us to read His Word exactly and work to understand what he is teaching. When we try to reason scripture out based on our own understanding, we take away the power of his word to cut to our soul.
Writings in the gospels sometimes don't line up because God did not intend them to. It is man's feable reasoning that says these quotes must match. If we understand his word or not, we should accept it as written and seek to understand the meaning. This is a much more rewarding way to read the Bible. God wrote what He wrote and there are specific messages there for us to discern. When we discover the intended meanings of difficult passages, there's great joy in the new understanding and increased faith in us of the wonder of our amazing God.