N. Leibowitz1 suggests that the differences are intentional changes made by Moshe so as to best get across his message to the new generation. Moshe purposefully presented the story in a way that would emphasize the nation's fault, ensuring that they learned to take full responsibility for their actions, and understood why they had spent forty years in the desert.2 This can account for many of the changes:
Initiator – Moshe highlights how the initial request came from the people, not Hashem, thereby preempting anyone from saying that they were just doing what God commanded.
Description of spies – In Devarim, the spies are not given names or titles to minimize their importance and the possibility of anyone attributing all blame to their leaders.
Positive or negative report? Moshe relays only the positive aspect of the spies' report, emphasizing instead the ensuing complaints of the nation.
Prayer – In Devarim, Moshe omits his prayer where he attempts to minimize the nation's sin and achieve forgiveness, since his goal is to highlight rather than hide the nation's mistakes.
Punishment – In Devarim, Moshe omits his prayer where he attempts to minimize the nation's sin and achieve forgiveness, since his goal is to highlight rather than hide the nation's mistakes.
R. Yaacov Medan3 accounts for the changes in the two books by positing that each is telling the story from a different perspective. He suggests that the spies were sent on a dual mission: a military reconnaissance mission as well as a surveying mission to determine the tribal inheritances. Sefer Devarim tells of the former, while Sefer Bemidbar focuses on the latter. Many of the differences are thus understandable:
Initiator – Hashem commanded the "holy" scouting mission so that the princes could evaluate the land and allocate it amongst the tribes, but the nation themselves initiated the spying mission in their desire to prepare for the conquest.
Who is sent? The scouting mission necessitated that the twelve princes of each tribe be chosen as representatives, while the military mission could have sufficed with anonymous men.
Moshe's instructions – In Bemidbar, Moshe tells the spies to appraise the quality of the land, its trees and agriculture, as the division of the land required such knowledge. In Devarim, he instead tells them to determine the route of conquest, crucial for their military strategy.
Purpose – The verb "לָתוּר" in Bemidbar highlights the scouting mission, whereas the verbs לחפר and לרגל in Devarim emphasize spying.
The scouted area – The survey required that the spies scout out the entire land as described in Bemidbar, while the military mission required them to spy only on the hilly region of Chevron, the original intended site of entry for the conquest.
Who is to blame? Moshe blames himself in Devarim because he realized that the mishap of the spies was largely due to his combining two missions which should have remained separate.
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