Passing an object variable vs. passing an object literal to a function in TypeScript

In TypeScript, if I define an interface MyInterface such as:

interface MyInterface {
  a ?: string;
}

And if I define a function myFunction that only accepts a parameter of type MyInterface such as:

function myFunction(parameter: MyInterface) {
  console.log("hello!")
}

If I define an object myObject

let myObject = {
  a: "lol",
  b: "hi"
}

And call myFunction with myObject as a parameter myFunction(myObject) TypeScript gives no errors.

But if I call myFunction with an object literal as a parameter with the same data as myObject

myFunction({a: "lol", b: "hi"}) TypeScript gives the following error:

Argument of type '{ a: string; b: string; }' is not assignable to parameter of type 'MyInterface'.
  Object literal may only specify known properties, and 'b' does not exist in type 'MyInterface'.(2345)

Why doesn't this error appear when I call myFunction with the object variable ?