vimproc#system() returns different result compared to builtin system()

[lambdalisue]

With vimproc 9.2, Vim 7.4.1246 in Ubuntu 15 (64bit).

echo printf('"%s"', system('echo @{upstream}'))
echo printf('"%s"', vimproc#system('echo @{upstream}'))

The code above show

"@{upstream}
"
"@u @p @s @t @r @e @a @m
"

As long as vimproc#system() is an alternative method of builtin system(), the result should be equal.
Additionally, I guess that you handle {} because of ${VARIABLE} context. If so you probably need to handle {} only when the leading character is $ (or is there anything what I don't know?)

[lambdalisue]

:h vimshell-tips-block

[Shougo]

Yes, it is feature.
And zsh same behaves.