Matthew 27:22 :
12 “What shall I do, then, with the one you call the king of the Jews?” Pilate asked them.
Note the (surprising) difference:“What shall I do, then, with Jesus who is called the Messiah?” Pilate asked.
They all answered, “Crucify him!”
In Mark the scribes and pharisees call Jesus as the king of the Jews.
In Matthew Jesus "is called Christ" obviously "by the people" but not by the scribes and pharisees (who want Jesus dead).
Matthew has omitted the embarrassing "fact" in Mark about who precisely called Jesus as "king of the Jews": the same his accusers.
If the same scribes and pharisees believed that Jesus is the Christ, then why did the same scribes and pharisees kill Jesus (by using Pilate)?
The more simple answer is that Jesus wasn't really the Christ, for Mark.
So the scribes and pharises appeal to Pilate because they want that Jesus seems really the Christ. But even Pilate isn't so sure about is messianic identity.
In Matthew the anonymous people call Jesus as "the Christ", but not the his accusers before Pilate.
So the reader is more persuaded that Jesus was the Christ, afterall, given the fact that only the his enemies didn't call him so.
So, if Jesus is not the Christ in Mark but was such for the his accusers, why then did the latter prefer Barabbas?
The Mark's irony is that they didn't know who was the father of Jesus, just when they were releasing a "Son of the Father" and putting to death their (only presumed) Christ.