Some emojis seem to be considered multiple characters. For example, the emoji is considered two, and if you try to get the first letter of it, it turns into � (which is apparently the “replacement glyph”).
This is a Unicode standard. Most emojis are two characters: A base character, like the square emoji, followed by a modifier character, like what color the square is.
You can check this with a character counting website if you want to verify for yourself.