grep -aPo "\x74\x79" and this doesn't? echo -ne "\x74\x79\xf4"

Question

为什么这个命令能用

echo -ne "\x74\x79\xf4"|grep -aPo "\x74\x79"

而这个却不行？

echo -ne "\x74\x79\xf4"|grep -aPo "\x74\x79\xf4"