Difference between assigning a value versus assigning another variable

Question

I have two objects p and q. Object q has one extra property 'b':

let p = {a:1}
let q = {a:1, b:2}

Now, if I assign q to p. No problem:

p = q           // ok 

However, the following has type error:

p = {a:1, b:2}  // not ok 

Error:

Type '{ a: number; b: number; }' is not assignable to type '{ a: number; }'.
  Object literal may only specify known properties, and 'b' does not exist in type '{ a: number; }'.
(2322)

Playground

In both cases, I am reassigning an object with extra properties to p. My understanding of duck-typing is that assigning objects are allowed as long as there is no missing property. But then why does the compiler complain in the latter case?