OCaml匹配元组？为什么未使用此匹配大小写？

let tuple = ("Hello", "World")

match tuple with
| ("hello", "world") -> print_endline "Matched lowercase"
| _ -> print_endline "No match"

let tuple = ("Hello", "World")

let lowercase_tuple = (String.lowercase_ascii (fst tuple), String.lowercase_ascii (snd tuple))

match lowercase_tuple with
| ("hello", "world") -> print_endline "Matched lowercase"
| _ -> print_endline "No match"

let tuple = ("Hello", "World")

match tuple with
| (s1, s2) when String.lowercase_ascii s1 = "hello" && String.lowercase_ascii s2 = "world" ->
    print_endline "Matched lowercase"
| _ -> print_endline "No match"