Why can attributes have missing values

From: Àî¼ÎÅô <lijpbasin_at_nyahnyahspammersnyahnyah>
Date: Thu Apr 26 2012 - 21:49:37 MDT

It's not a big issue, just I have to write the if statements like this:
if (.not. ismissing(var@var_type) .and. var@var_type .eq. "upl") then
......

It's shown by the example below:
v = 1

v@a = "missing"
print(v)
print(v@a)
print(ismissing(v@a))

v@a = new(1, string)
print(v)
print(v@a)
print(ismissing(v@a))

The two "print(v)" and "print(v@a)" produced the same output while the two "print(ismissing(v@a))" produced different output.
It looked as if the attribute "a" of variable "v" still have an _FillValue attribute, which is not included in the NCL variable model (as I perceived). Is it a desired behavior or just an accident?
# uname -a
Linux xxx 2.6.18-164.el5 #1 SMP Tue Aug 18 15:51:48 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux
# ncl -V
6.0.0

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Received on Thu Apr 26 21:49:56 2012

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