LC_ALL = Cの有無にかかわらず、 `grep -o`の出力が同じ理由は何ですか？

grep -oLC_ALL = Cの有無にかかわらず、出力が同じ理由は何ですか？予想どおり、フラグなしのgrepは違いがありますがgrep -o...常にgrep -oLC_ALL = Cまたは他のものを使用しますか？

[aa@bb grep-test]$ cat input.txt
aa bb
CC cc
dd ee

[aa@bb grep-test]$ LC_ALL=C grep -o [A-Z] input.txt
C
C
[aa@bb grep-test]$ grep -o [A-Z] input.txt
C
C
[aa@bb grep-test]$ LC_ALL=C grep [A-Z] input.txt
CC cc
[aa@bb grep-test]$ grep [A-Z] input.txt
aa bb
CC cc
dd ee
[aa@bb grep-test]$ grep -V
GNU grep 2.6.3
...
[aa@bb src]$ ./grep -V
grep (GNU grep) 2.27
...
[aa@bb src]$ ./grep [A-Z] ../../test
CC cc
[aa@bb src]$ 
[aa@bb grep-test]$ grep a input.txt
aa bb
[aa@bb grep-test]$ grep C input.txt
CC cc
[aa@bb grep-test]$ locale
LANG=en_IE
LC_CTYPE="en_IE"
LC_NUMERIC="en_IE"
LC_TIME="en_IE"
LC_COLLATE="en_IE"
LC_MONETARY="en_IE"
LC_MESSAGES="en_IE"
LC_PAPER="en_IE"
LC_NAME="en_IE"
LC_ADDRESS="en_IE"
LC_TELEPHONE="en_IE"
LC_MEASUREMENT="en_IE"
LC_IDENTIFICATION="en_IE"
LC_ALL=
[aa@bb grep-test]$ bash --version
GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)
...
[aa@bb grep-test]$ xxd input.txt
0000000: 6161 2062 620a 4343 2063 630a 6464 2065  aa bb.CC cc.dd e
0000010: 650a 0a                                  e..
[aa@bb grep-test]$ cat -A input.txt
aa bb$
CC cc$
dd ee$
$
[aa@bb grep-test]$