違法な番号を受け取るのはなぜですか？ {1..3}

私はなぜ受けますか？

./6_sum_square_difference.sh: 11: ./6_sum_square_difference.sh: Illegal number: {1..3}

~のため

#!/bin/sh
summing () {
  the_total=0
  num_in=$1
  for one_num in {1..$num_in}
  do  
    printf "aaaa\n"
    the_total=$((the_total+one_num)) # <-- Line 11
  done
}
summing 3
if [[ $the_total == 6 ]]; then
  echo "equa to 6 "$the_total
else
  echo "NOT equal to 6"
fi
printf "total= "$the_total