Criterion Weirdness

Main.hs

{-
Maxwell-0.0.0: benchmarks
Running 1 benchmarks...
Benchmark Maxwell-benchmarks: RUNNING...
benchmarking fib/30
time                 51.14 ms   (49.98 ms .. 52.85 ms)
                     0.994 R2   (0.977 R2 .. 1.000 R2)
mean                 51.82 ms   (51.06 ms .. 53.95 ms)
std dev              2.150 ms   (1.031 ms .. 3.451 ms)
variance introduced by outliers: 14% (moderately inflated)

benchmarking fib/30
time                 50.76 ms   (50.45 ms .. 50.98 ms)
                     1.000 R2   (1.000 R2 .. 1.000 R2)
mean                 50.84 ms   (50.64 ms .. 51.15 ms)
std dev              430.5 s   (240.5 s .. 728.9 s)

benchmarking fib/30
time                 50.72 ms   (50.39 ms .. 51.04 ms)
                     1.000 R2   (1.000 R2 .. 1.000 R2)
mean                 50.87 ms   (50.68 ms .. 51.12 ms)
std dev              418.5 s   (261.5 s .. 606.5 s)

benchmarking fib/nub
time                 50.75 ms   (50.56 ms .. 50.96 ms)
                     1.000 R2   (1.000 R2 .. 1.000 R2)
mean                 50.97 ms   (50.73 ms .. 51.60 ms)
std dev              681.4 s   (220.0 s .. 1.151 ms)

Benchmark Maxwell-benchmarks: FINISH
Completed 2 action(s).
-}

-- You can benchmark your code quickly and effectively with Criterion. See its
-- website for help: <http://www.serpentine.com/criterion/>.
import Criterion.Main

fib :: Integer -> Integer
fib m | m < 0     = error "negative!"
      | otherwise = go m
  where go 0 = 0
        go 1 = 1
        go n = go (n-1) + go (n-2)

main :: IO ()
main = defaultMain [
  bgroup "fib" [ bench "30"  $ whnf fib 30
               , bench "30"  $ whnf fib 30
               , bench "30"  $ whnf fib 30
               , bench "30" $ whnf fib 30
               ]
  ]