Waiting for garbage collection can kill parallelism?

Today I was reading "Parallel Performance Tuning for Haskell" by Jones, Marlow and Singh and 
wanted to replicate the results for their first case study. The code goes like this:

module Main where
import Control.Parallel

main :: IO ()
main = print . parSumFibEuler 38 $ 5300

parSumFibEuler :: Int -> Int -> Int
parSumFibEuler a b = f `par` (e `pseq` (e + f))
    where f = fib a
          e = sumEuler b

fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib (n - 1) + fib (n - 2)

mkList :: Int -> [Int]
mkList n = [1..n-1]

relprime :: Int -> Int -> Bool
relprime x y = gcd x y == 1

euler :: Int -> Int
euler n = length (filter (relprime n) (mkList n))

sumEuler :: Int -> Int
sumEuler = sum . (map euler) . mkList