How does Scala use all my cores here?

object PrefixScan {
  sealed abstract class Tree[A]
  case class Leaf[A](a: A) extends Tree[A]
  case class Node[A](l: Tree[A], r: Tree[A]) extends Tree[A]

  sealed abstract class TreeRes[A] { val res : A }
  case class LeafRes[A](override val res: A) extends TreeRes[A]
  case class NodeRes[A](l : TreeRes[A], override val res: A, r: TreeRes[A]) extends TreeRes[A]

  def reduceRes[A](t: Tree[A], f:(A,A)=>A): TreeRes[A] = t match {
    case Leaf(v) => LeafRes(v)
    case Node(l, r) => {
      val (tL, tR) = (reduceRes(l, f), reduceRes(r, f))
      NodeRes(tL, f(tL.res, tR.res), tR)
    }
  }
}

I'm concerned about the reduceRes function.

It works ... the result of the computation is great!

However I went and implemented another version, reduceResPar, that uses fork-join at the first few branches, to parallelize the computation. But it gave no speed up.

Then I went back and realized .. the above version, reduceRes, is already using all 12 cores on my machine!! How can it do that? I thought it would just be 1 core!

This code is from the Parallel Programming course on Coursera In the last lecture of week 2, we are learning about parallel prefix scan operations.