求助~最大公约数脚本，大小随意赋值给变量的问题

advnced bash中的一个例子提到

我的推导过程
dividend=10
divisor=20
until里面的运算步骤为
remainder=$dividend % $divisor=10 % 20 =0
dividend=$divisor=20
remainder等于0了，循环结束，那么结果为$dividend=20?
明显不对呀？

附上脚本源码
#!/bin/bash

ARGS=2
E_BADARGS=65

if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` first-number second-number"
exit $E_BADARGS
fi

gcd ()
{
dividend=$1
divisor=$2

remainder=1

until [ "$remainder" -eq 0 ]
do
let "remainder = $dividend % $divisor"
dividend=$divisor
divisor=$remainder
done
}

gcd $1 $2

echo; echo "GCD of $1 and $2 = $dividend"; echo

exit 0