[Avodah] Why Not: Yehoshua BEN Nun?

[cantorwolberg at cox.net]

Has anyone ever wondered why in the Torah Joshua is referred to as "BIN Nun" as opposed to Ben Nun?
I have seen the attempted explanations, none of which are satisfactory (to me).
All of the other spies are referred to as ploni BEN ploni and Joshua is the only one referred to as Yehoshua BIN Nun.     
Why is a chirik used instead of a segol.
One explanation by the Ramban (In his commentary to Sh'mos 33:11) is that the two words should be read as one: "binnun."
He says it relates to the word "binah" because of Joshua's sharp intelligence.
After reading that explanation, it seems obvious to me that nobody knows why and perhaps it is an error.