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That confusing snippet was changed in newer versions of GNU grep to:

-i, -ignore-case Ignore case distinctions, so that characters that differ only in case match each other.

See this commit: http://git.savannah.gnu.org/cgit/grep.git/commit/?id=e1ca01be48cb64e5eaa6b5b29910e7eea1719f91

 .BR \-i ", " \-\^\-ignore\-case
-Ignore case distinctions in both the
-.I PATTERN
-and the input files.
+Ignore case distinctions, so that characters that differ only in case
+match each other.

As to where the old formulation may originate, some programs like less(1) have a (mis)feature where using an uppercase letter in a pattern will turn off case insensitivity for a particular search (override the -i flag). The author of that doc snippet probably assumed that many people expected that behavior, and instead of some direct caveat, preferred that non-committal sentence. FWIW, such a feature was never a part of ed(1), grep(1), vi(1), perl(1) etc. or of the regex(3) or pcre(3) APIs.

That confusing snippet was changed in newer versions of GNU grep to:

-i, -ignore-case Ignore case distinctions, so that characters that differ only in case match each other.

See this commit: http://git.savannah.gnu.org/cgit/grep.git/commit/?id=e1ca01be48cb64e5eaa6b5b29910e7eea1719f91

 .BR \-i ", " \-\^\-ignore\-case
-Ignore case distinctions in both the
-.I PATTERN
-and the input files.
+Ignore case distinctions, so that characters that differ only in case
+match each other.

As to where the old formulation may originate, some programs like less(1) have a (mis)feature[1] where using an uppercase letter in a pattern will turn off case insensitivity for a particular search (override the -i flag). The author of that doc snippet probably assumed that many people expected that behavior, and instead of some direct caveat, preferred that non-committal sentence. FWIW, such a feature was never a part of ed(1), grep(1), vi(1), perl(1) etc. or of the regex(3) or pcre(3) APIs.

[1] that seems to have its origins in emacs, where it's the default; there you can turn it off by setting the (customizable) search-upper-case variable to nil.

That confusing snippet was changed in newer versions of GNU grep to:

-i, -ignore-case Ignore case distinctions, so that characters that differ only in case match each other.

See this commit: http://git.savannah.gnu.org/cgit/grep.git/commit/?id=e1ca01be48cb64e5eaa6b5b29910e7eea1719f91

 .BR \-i ", " \-\^\-ignore\-case
-Ignore case distinctions in both the
-.I PATTERN
-and the input files.
+Ignore case distinctions, so that characters that differ only in case
+match each other.

As to where the old formulation may originate, some programs like less(1) have a mis-feature where using an uppercase letter in a pattern will turn off case insensitivity for a particular search (override the -i flag). The author of that doc snippet probably assumed that many people expected that behavior, and instead of some direct caveat, preferred that non-committal sentence. FWIW, such a mis-feature was never a part of ed(1), grep(1), vi(1), perl(1) etc. or of the regex(3) or pcre(3) APIs.

That confusing snippet was changed in newer versions of GNU grep to:

-i, -ignore-case Ignore case distinctions, so that characters that differ only in case match each other.

See this commit: http://git.savannah.gnu.org/cgit/grep.git/commit/?id=e1ca01be48cb64e5eaa6b5b29910e7eea1719f91

 .BR \-i ", " \-\^\-ignore\-case
-Ignore case distinctions in both the
-.I PATTERN
-and the input files.
+Ignore case distinctions, so that characters that differ only in case
+match each other.

As to where the old formulation may originate, some programs like less(1) have a (mis)feature where using an uppercase letter in a pattern will turn off case insensitivity for a particular search (override the -i flag). The author of that doc snippet probably assumed that many people expected that behavior, and instead of some direct caveat, preferred that non-committal sentence. FWIW, such a feature was never a part of ed(1), grep(1), vi(1), perl(1) etc. or of the regex(3) or pcre(3) APIs.

That confusing snippet was changed in newer versions of GNU grep to:

-i, -ignore-case Ignore case distinctions, so that characters that differ only in case match each other.

See this commit: http://git.savannah.gnu.org/cgit/grep.git/commit/?id=e1ca01be48cb64e5eaa6b5b29910e7eea1719f91

 .BR \-i ", " \-\^\-ignore\-case
-Ignore case distinctions in both the
-.I PATTERN
-and the input files.
+Ignore case distinctions, so that characters that differ only in case
+match each other.

That confusing snippet was changed in newer versions of GNU grep to:

-i, -ignore-case Ignore case distinctions, so that characters that differ only in case match each other.

See this commit: http://git.savannah.gnu.org/cgit/grep.git/commit/?id=e1ca01be48cb64e5eaa6b5b29910e7eea1719f91

 .BR \-i ", " \-\^\-ignore\-case
-Ignore case distinctions in both the
-.I PATTERN
-and the input files.
+Ignore case distinctions, so that characters that differ only in case
+match each other.

As to where the old formulation may originate, some programs like less(1) have a mis-feature where using an uppercase letter in a pattern will turn off case insensitivity for a particular search (override the -i flag). The author of that doc snippet probably assumed that many people expected that behavior, and instead of some direct caveat, preferred that non-committal sentence. FWIW, such a mis-feature was never a part of ed(1), grep(1), vi(1), perl(1) etc. or of the regex(3) or pcre(3) APIs.