First of all, the Catechism of the Council of Trent, an authoritative source which explains:
The additional words for you and for many, are taken, some from Matthew, some from Luke, but were joined together by the Catholic Church under the guidance of the Spirit of God. They serve to declare the fruit and advantage of His Passion. For if we look to its value, we must confess that the Redeemer shed His blood for the salvation of all; but if we look to the fruit which mankind have received from it, we shall easily find that it pertains not unto all, but to many of the human race. When therefore ('our Lord) said: For you, He meant either those who were present, or those chosen from among the Jewish people, such as were, with the exception of Judas, the disciples with whom He was speaking. When He added, And for many, He wished to be understood to mean the remainder of the elect from among the Jews or Gentiles.For more discussion, you can read the articles written by Fr Z for the Wanderer and posted on his blog:
With reason, therefore, were the words for all not used, as in this place the fruits of the Passion are alone spoken of, and to the elect only did His Passion bring the fruit of salvation. And this is the purport of the Apostle when he says: Christ was offered once to exhaust the sins of many; and also of the words of our Lord in John: I pray for them; I pray not for the world, but for them whom thou hast given me, because they are thine.