Rashi tells that the Tochechah in our sidra is Mi-pi HaG’vurah, from the mouth of God, and the one in Ki Tavo is Mi-pi Atzmo, from the mouth of Moses.
Rav Soloveitchik asks how this distinction is possible when the whole Torah is the word of the Almighty.
His answer is that in our sidra Moses is the representative of Israel. The words come directly from HaShem with Moses in the front line of the recipients.
In Ki Tavo, God, as it were, gives Moses a vote of confidence. He tells him that by now he has enough experience and expertise to be trusted to select the right style and phraseology to convey the message. There is no difference in the ultimate source of the words, nor their authority.
On both occasions they come from HaShem. The difference is in the way in which they are conveyed. Leadership has matured and enriched Moses.
The task has not been easy. You can see this on Moses’ face, as you can see how life and experience weather any leader after some years. But now, if I may offer a personal analogy, the Commander knows His agent can handle the responsibility that lies in his hands.
The analogy, l’havdil is this. Years ago I headed a department in the Office of the Chief Rabbi in London. After a while the Chief was apparently sufficiently satisfied to say, “This is what I want you to say on my behalf; you know my style, so you choose the words for me.”