bash子串扩展不对

Question

子串扩展的工作方式与预期不符。

a='123?'
echo $a         #outputs '123?' as expected
echo ${a:3:1}   #outputs '=' not as expected
echo ${a:2:2}   #outputs '3?' as expected
echo "${a:3:1}" #outputs '?' as expected

我无法理解，请帮助我。