In Matthew and Mark, Jesus is asked by James, John, and their mother for the positions of sitting at the right and left hand of Jesus in His kingdom. He responds by saying that He does not have the authority to grant their request, because the Father has determined, or will determine, who will occupy those positions:
Matthew 20:20-23 (NRSVUE)
20 Then the mother of the sons of Zebedee came to him with her sons, and kneeling before him, she asked a favor of him.
21 And he said to her, "What do you want?" She said to him, "Declare that these two sons of mine will sit, one at your right hand and one at your left, in your kingdom."
22 But Jesus answered, "You do not know what you are asking. Are you able to drink the cup that I am about to drink?" They said to him, "We are able."
23 He said to them, "You will indeed drink my cup, but to sit at my right hand and at my left, this is not mine to grant, but it is for those for whom it has been prepared by my Father."
The parallel to the above account is in Mark 10:35-40.
Consider this account from a Trinitarian perspective, or a perspective which maintains that Jesus is God. In that case, Jesus actually did determine (or will determine) who will sit at His right and left hand, because He is the very same Being as God. If there is one God, and God determines these positions, then if Jesus is God, He certainly determined them.
If Jesus did not determine them despite being God, then there would be two distinct wills within God. And, one of those wills could make a decision, and yet the Being (God) who holds those wills could still in some sense say that He did not make it, and even that the decision was not His to make. Is this a coherent view? Is there anything in the context of these passages to suggest that such a difficult interpretation is to be sought after?
Rather, this account is presented in a way that is very easy to understand if in fact the Father is one Being, and Jesus is another, as Unitarianism teaches. That is why Jesus can say that He did not have the authority to grant a position in His kingdom, and why neither He nor the writers of the Gospels felt any need to explain or caveat such a statement.