The LEFT formula takes two arguments: the thing you're taking the left-hand side of, and the number of characters you want to take. So the following formula takes the first five characters of cell A1.
But if you only need one character, there's no need for a second argument. So the following formula will take the first character of cell A1.
The lack of a second argument means that Excel defaults to taking a single character. And the same is true of the RIGHT function.