Why undeclared field in union type can be filled with declared field type

Question

code as follow

My expected is if b is not declared, there should be no b in the assigned value.

My another expected is if b is not declared, there should be any b in the assigned value， but also c d ...

type Target =
  | {
      a: 1;
    }
  | {
      a: 2;
      b: number;
    }
  | {
      a: 3;
      b: boolean;
    };


// The result is 

const target: Target = {
  a: 1,
  // correct
};

const target: Target = {
  a: 1,
  b: 1, // correct
};

const target: Target = {
  a: 1,
  b: "true", // incorrect
};

const target: Target = {
  a: 1,
  b: true, // correct
};

// My expected result is

const target: Target = {
  a: 1,
  // correct
};

const target: Target = {
  a: 1,
  b: 1, // incorrect
};

const target: Target = {
  a: 1,
  b: "true", // incorrect
};

const target: Target = {
  a: 1,
  b: true, // incorrect
};


// My another expected result is

const target: Target = {
  a: 1,
  // correct
};

const target: Target = {
  a: 1,
  b: 1, // correct
};

const target: Target = {
  a: 1,
  b: "true", // correct
};

const target: Target = {
  a: 1,
  b: true, // correct
};

const target: Target = {
  a: 1,
  c: true, // correct
};

How can this be completed?