In short
Jesus marveled not at his own power to heal, for he had done so many times, but at the great faith of the centurion in Jesus’ power to heal with only a word and Jesus’ willingness to do so.
Jesus most certainly did not marvel at the fact that the healing took place, for this was quite normal to him. What was uncommon for him was finding someone with such great faith in him. The centurion's great faith refers to the trust he had in the power of Jesus: he believed that Jesus would be willing to help him as a Gentile; he believed that Jesus has the power to heal even if he wasn’t in the patient’s presence, and he believed that Jesus can heal by simply speaking a word.1
10 When Jesus heard this, he marveled and said to those who followed him, “Truly, I tell you, with no one in Israelhave I found such faith.