[Mesorah] trop differences at the beginning of Sh'lach

[Sholom Simon via Mesorah]

The structure of 11 of 12 of the p'sukim (from 13:5-16) are identical:

	l'mateh [name of sheve] ploni ben-almoni

(One is different because it's "l'mateh Yosef l'mateh Menasha)

And the trop for each of those 11 psukim are identical:

	munach pashta tipcha sof-pasuk

Except for one of them, at 13:8 ('l'mateh Efrayim Hoshei bin-Nun"), 
where the trop is:

	mercha tipcha mercha sof-pasuk

Why are 10 the psukim one way, and the 11th one different?  (For that 
matter, with my limited knowledge,  it seems the trop at 13:8 makes 
much more sense to me -- why aren't all of them like that one?)

(It does occurs to me that the two puskim of the 12 that are 
different are those of shevatim Menashe and Efrayim -- but that 
strikes me as drash rather than dikduk)

Thoughts?

-- Sholom