+RTS -s reports low maximum residency when using the non-moving GC

I'm trying to reproduce some of these GC benchmarks using the classic "Pusher" program.

The code

module Main (main) where

import qualified Control.Exception as Exception
import qualified Control.Monad as Monad
import qualified Data.ByteString as ByteString
import qualified Data.Map.Strict as Map

data Msg = Msg !Int !ByteString.ByteString

type Chan = Map.Map Int ByteString.ByteString

_N :: Int
_N = 500 * 1000

message :: Int -> Msg
message n = Msg n (ByteString.replicate 1024 (fromIntegral n))

pushMsg :: Chan -> Msg -> IO Chan
pushMsg chan (Msg msgId msgContent) =
  Exception.evaluate $
    let
      inserted = Map.insert msgId msgContent chan
    in
      if _N < Map.size inserted
      then Map.deleteMin inserted
      else inserted

main :: IO ()
main = Monad.foldM_ pushMsg Map.empty (map message [1..2000000])

I compile the program with ghc -threaded -rtsopts -O2 pusher.hs using GHC-8.10-alpha1 from hvr's PPA and run it:

$ ./pusher +RTS -xn -s
   6,524,460,200 bytes allocated in the heap
     510,389,120 bytes copied during GC
     392,148,056 bytes maximum residency (23 sample(s))
     109,980,584 bytes maximum slop
            1288 MiB total memory in use (0 MB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0      6822 colls,     0 par    0.826s   0.518s     0.0001s    0.0005s
  Gen  1        23 colls,     0 par    0.132s   0.131s     0.0057s    0.0177s

  TASKS: 27 (24 bound, 3 peak workers (3 total), using -N1)

  SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

  INIT    time    0.001s  (  0.001s elapsed)
  MUT     time    1.780s  (  1.212s elapsed)
  GC      time    0.958s  (  0.649s elapsed)
  EXIT    time    0.074s  (  0.079s elapsed)
  Total   time    2.813s  (  1.940s elapsed)

  Alloc rate    3,665,265,491 bytes per MUT second

  Productivity  63.3% of total user, 62.4% of total elapsed

Since the program builds a Map of 500,000 messages of 1024 bytes each, I'd expect a maximum residency of > 500MB. But the RTS reports only slightly less than 400MB.

If I use the default GC, the RTS reports a more reasonable 588MB.