Why was this particular set of tribes chosen to stand on Mount Gerizim and the other six on Mount Eval, rather than some other combination of the tribes?
(27,12) “These shall stand on Mount Gerizim to bless the people when you pass over the Jordan: Shimon and Levi and Yehudah and Yissachar and Yosef and Binyamin. And these shall stand on Mount Eval for the curse: Reuven, Gad and Asher, and Zevulun, Dan and Naftali.”
The magazine Tradition records that two mathematicians, Rabbi Michael Broyde and Steven Weiner, wanted to know why Hashem specifically chose these two sets of tribes to stand on Mount Gerizim and Mount Eval, rather than some other combination. Pursuing this question they decided to run a computer program which calculated all the possible combinations of the tribes into two sets of six, together with the number of men who would be in each set according to the population numbers from the earlier census.
What emerged from their study is quite remarkable. Only with the combination that the Torah writes were the number of men on the two mountains almost exactly equal (there was a difference of one), whereas all the other combinations resulted in the number of men on one mountain clearly greater than the number of men on the other mountain!
When I read about this study I commented that presumably Hashem did this in order that Yisrael would have free will to make a choice between the blessings and the curses (to choose between doing good or doing bad). Because if there was a majority on one of the mountains this could influence a person to follow that majority.
But the problem with this explanation is that there was in fact a majority on one of the mountains - Mount Eval, albeit a very small majority of one man. However, I immediately realized that this is not true, because the gemora in Sanhedrin 2a teaches that when the majority is for the side of good then even when there is only a majority of one it is considered a majority, but when the majority is for the side of bad then a majority of at least two is needed to be considered a majority.
For this reason Hashem organized the choice of tribes so that the majority of one would be on Mount Eval, the mountain for the curses. Thus, since this was the side of bad this majority of one was not considered a majority and thus Yisrael had the freewill to choose between good and bad. Remarkable indeed!