{Bereishis 13:16}
“I will make your offspring as the dust of the earth so that if one can count the dust of the earth, then your offspring, too, can be counted.”
Rashi comments: Just as it is impossible for the dust to be counted, so your offspring will not be counted.
As Rashi explains, the blessing is that Bnei Yisroel will not be able to be counted, like the dust. Why then doesn’t the posuk write it that way – “just as dust cannot be counted so too your offspring will not be able to be counted”? Why is it written in the positive, thereby implying the opposite of the blessing’s intent?
If the inability to count the Jewish Nation is a sign of their greatness and physical mass, then perhaps I can offer an answer to this wonderful question.
As it is well known that the allusion to dust is an allusion to a time when the Children of Avraham, are not at their spiritual prime, perhaps, in the blessing, the Torah alludes to a particular vulnerabilities.
Good Luck with this wonderful project and kudos on the poignant textual questions.
There is a similar question based on the haftorah to parshas bamidbar. See here: http://thoughtsonparsha.blogspot.com/2012_05_01_archive.html