Why in a generic function, if the param is or not is optional, will cause the same call operation to inference different type?

Question

For example the following code:

function a<T extends string | undefined>(value?: T): T {
    return value as T;
}

function b<T extends string | undefined>(value: T): T {
    return value;
}

const val = 'value' as string | undefined;

const i = a(val);
const j = b(val);

ant it's type is:

declare function a<T extends string | undefined>(value?: T): T;
declare function b<T extends string | undefined>(value: T): T;

declare const val: string | undefined;

declare const i: string;
declare const j: string | undefined;

so, why i's type is string, and j's type is string | undefined.