Oznayim la'Torah: They are not the same incident. Refer to Bamidbar, 27:13:152:2*. The Torah there uses a Lashon of 'Vayomer' (denoting Midas Rachamim) since it was not connected to Moshe's death and was purely an act of Rachamim, as opposed to the current Pasuk, where he was told that he would die on the same day, and that all his Tefilos to remain alive in different forms had been refuted. Consequently, the Lashon 'Dibur' (denoting Midas ha'Din) is more appropriate.

Rashi (citing the Sifri): Because Yisrael were saying that they would never allow Moshe to be taken away from them. ¹ So Hakadosh-Baruch-Hu said 'I will take him away in the middle of the day; See if you can stop Me!' ²

Targum Yonasan: To hint that he would die on that very day - the seventh of Adar, the same day on which he was born. ³

Kli Yakar: Death is called 'sunset'. Because Moshe at the time of his death still had his full strength; it is as if he was in the middle of his days, like the sun when it is at its height - at noon.