$INSTDIR is empty in directory page show callback function

With the following codes, $INSTDIR is empty in
directoryShow function, while it is "C:\Test" in
directoryPre function.

Name "Test"

OutFile "Test.exe"

InstallDir "C:\Test"

PageEx directory
PageCallbacks directoryPre directoryShow
PageExEnd
Page components
Page instfiles

Function directoryPre
MessageBox MB_OK "directoryPre: $INSTDIR"
FunctionEnd
Function directoryShow
MessageBox MB_OK "directoryShow: $INSTDIR"
FunctionEnd

Section
SectionEnd