Why does performGC fail to release all memory?

Given the program:

import Language.Haskell.Exts.Annotated -- from haskell-src-exts
import System.Mem
import System.IO
import Control.Exception

main :: IO ()
main = do
  evaluate $ length $ show $ fromParseResult $ parseFileContents $ "data C = C {a :: F {- " ++ replicate 400000 'd' ++ " -}     }"
  performGC
  performGC
  performGC

Using GHC 7.0.3, when I run:

$ ghc --make Temp.hs -rtsopts && Temp.exe +RTS -G1 -S
    Alloc    Copied     Live    GC    GC     TOT     TOT  Page Flts
    bytes     bytes     bytes  user  elap    user    elap
 ...
 29463264        64   8380480  0.00  0.00    0.64    0.85    0    0  (Gen:  0)
       20        56   8380472  0.00  0.00    0.64    0.86    0    0  (Gen:  0)
        0        56   8380472  0.00  0.00    0.64    0.87    0    0  (Gen:  0)
    42256       780     33452  0.00  0.00    0.64    0.88    0    0  (Gen:  0)
        0                      0.00  0.00

The performGC call seems to leave 8Mb of memory live, even though it seems like all the memory should be dead. How come?

(Without -G1 I see 10Mb live at the end, which I also can't explain.)