[Avodah] Rashi on kol hanefesh (Bereshis 46:26)

[Sholom Simon]

Kol hanefesh . . . shvi'im.  Rashi says "I found in Vayikra Rabba" . 
. . a statement that Eisav had (with him) six, and the word nafashos 
(plural) was used; but here with Yaakov, kol hanafesh . . . shevi'im, 
nefesh is in the singular.  This is a hint that Eisav engaged in A"Z 
(plural gods), while Yaakov served one G-d.

But don't we (almost?) always see (in Torah) the singular being used 
for a noun when the count is larger than 10?  How can one darshen 
from something that follows the general grammatic rule?

What am I missing here?

-- Sholom