fconfigure oddness

#!/usr/bin/tclsh
#
# the files I am reading are shell scripts with tgz appended to them,
# intended to be read by "tail -n +100 | tar xzv".  Why do I need to
# [seek $f [tell $f]] after [chan configure]?
#
foreach fn [glob *.exe] {
    set basename [lindex [split $fn _.] 1]
    file mkdir $basename

    set f [open $fn]
    while {[set l [gets $f]] != "exit"} {}

    chan configure $f -encoding binary -translation binary

    # needing to do this is a bit odd
    seek $f [tell $f]

    exec tar -C $basename -xzv <@ $f

    close $f
}