Could optional types be more specific?

[some-user123]

I'm under the impression that generated types are to broad and as specific as they could be.

As an example, I'm having in a graphql schema like this:

type Example {
  optional: String
  mandatory: String!
}

If I query this entity, the generated type is:

// ...
    mandatory: string;
    optional?: string | null | undefined;
// ...

From my (limited) understanding of GraphQL that could be

    optional: string | null;

because the field was requested, so it has to be contained in the answer...

Under which condition would that field be undefined?