Parentheses inside an anonymous function with a vector as a parameter

I am still learning clojure and I have doubts.

when I write an anonymous function, I do it

fn [parameter] 
 (do something)

ok .. function body is enclosed in parentheses

now i'm reading a fibonacci solution like this

(iterate (fn [[a b]] [b (+ a b)]) [0 1]))

My doubt is why I don't like it

(iterate (fn [[a b]]  (    [b (+ a b)]   ))   [0 1]))

I turn on the function ()

(iterate (fn [[a b]]   "("    [b (+ a b)]   ")"  )   [0 1]))

it receives a vector and then returns a function of the body than another vector ...

This is different when I call a function with a vector parameter or I make a big mistake.