Hello and welcome,

it’s because they are the same thing…

a charlist is a list of char.

iex> [97, 98, 99]
'abc'
iex> [97, 98, 99, 0]
[97, 98, 99, 0]
iex> [97, 98, 99] == 'abc'
true
iex> Enum.map 'abc', & IO.inspect(&1)
97
98
99
'abc'

If it can, it will output char

For information how to change this behaviour, see this previous answer: IEX - char printing - odd behavior

As other said, in addition You can use i/1 helper function in IEx session to get more informations about given type:

iex(1)> i [7,8,9]
Term
  '\a\b\t'
Data type
  List
Description
  This is a list of integers that is printed as a sequence of characters
  delimited by single quotes because all the integers in it represent printable
  ASCII characters. Conventionally, a list of Unicode code points is known as a
  charlist and a list of ASCII characters is a subset of it.
Raw representation
  [7, 8, 9]
Reference modules
  List
Implemented protocols
  Collectable, Enumerable, IEx.Info, Inspect, List.Chars, String.Chars

Because that’s the way Erlang represents strings. It makes interaction a lot easier when errors from Erlang are prettyprinted as charlists by default rather than as a list of strings.

It’s not really treating them differently, because they are charlists. As charlists are just lists of integers that represent characters, there is no way to differentiate them from “normal” lists. IEx is the one treating them differently when outputting them, as it tries to be friendly to you.

There is a similar thing with binaries, where strings are just binaries that contain UTF-8 data. IEx will print those binaries as strings. If there is data that is not valid UTF-8 or is not printable, it will print the bytes as numbers.