tcl vs system copy

Hi How come copy -f [glob -directory ${filespath} common_*.tiff] \ ${destroot}${prefix}/$gslibrary/Images and file copy -force [glob -directory ${filespath} common_*.tiff] \ ${destroot}${prefix}/$gslibrary/Images do not work while system "cp -f ${filespath}/common_*.tiff \ ${destroot}${prefix}/$gslibrary/Images" does ? yves