pipe password work with a but not with x

[Mario Pilesi]

echo pwd|"C:\Program Files\7-Zip\7z.exe" a file.7zx k:\file.au3 -p
work
echo pwd|"C:\Program Files\7-Zip\7z.exe" x "K:\file.7zx" -oK:\file -p
not work
why?
i would to avoid pass password in command line directly with -p parameter for best sure.