Adam Clarke on Deuteronomy 27:12

These shall stand upon Mount Gerizim to bless the people - Instead of upon Mount, etc., we may translate by, as the particle על al is sometimes used; for we do not find that the tribes did stand on either mount, for in Jos 8:33, when this direction was reduced to practice, we find the people did not stand on the mountains, but over against them on the plain. See the observations at the end of this chapter, Deu 27:26 (note).