Inconsistent allocation stats

I'm looking at Criterion internals, and seeing an inconsistency in the allocations reported by GCStats and RTSStats. Here is a small reproduction:

{-# LANGUAGE CPP #-}
module Main where

import GHC.Stats
import System.Mem (performGC)

main :: IO ()
main = do
    runOldThing 1000
#if __GLASGOW_HASKELL__ >= 802
    putStrLn "Running new:"
    runThing 1000
#endif

runOldThing :: Int -> IO ()
runOldThing n = loop n 0 >> return ()
  where
    loop 0 _ = return 0
    loop count x = do
      performGC
      stats <- getGCStats
      putStrLn $ show (count `mod` 15) ++ ": " ++ show (bytesAllocated stats - x) ++ " num: " ++ show (numGcs stats)
      loop (count-1) (bytesAllocated stats)

#if __GLASGOW_HASKELL__ >= 802
runThing :: Int -> IO ()
runThing = loop
  where
    loop 0 = return ()
    loop n = do
      performGC
      stats <- getRTSStats
      putStrLn $ show (n `mod` 15) ++ ": " ++ show (gcdetails_allocated_bytes (gc stats)) ++ " num: " ++ show (gcs stats)
      loop (n-1)
#endif

This code just performs a garbage collection and then prints the stats in a loop. Here is a snippet of the output.

...
4: 8840 num: 1967
3: 4880 num: 1968
2: 4880 num: 1969
1: 4880 num: 1970
0: 4880 num: 1971
14: 4880 num: 1972
13: 4976 num: 1973
12: 4976 num: 1974
11: 4976 num: 1975
10: 4976 num: 1976
9: 4976 num: 1977
8: 4880 num: 1978
7: 4880 num: 1979
6: 4880 num: 1980
5: 4880 num: 1981
4: 8840 num: 1982
3: 4880 num: 1983
2: 4880 num: 1984
1: 4880 num: 1985
0: 4880 num: 1986
14: 4880 num: 1987
13: 4976 num: 1988
12: 4976 num: 1989
11: 4976 num: 1990
10: 4976 num: 1991
9: 4976 num: 1992
8: 4880 num: 1993
7: 4880 num: 1994
6: 4880 num: 1995
5: 4880 num: 1996
4: 8840 num: 1997
3: 4880 num: 1998
2: 4880 num: 1999
1: 4880 num: 2000

On the left, I've included the gc number mod 15 to show that exactly every 15 gcs, there is an extra 4k bytes reported. This output was made with 8.2.1. On 7.8.4, 7.10.3, and 8.0.2 it's every 23. And on 8.4.0.20180204 it's every 14.

I've played around with extra allocations between garbage collections, but the interval remained constant. I tried poking around the rts, but I've been unable to determine if this is a bug or just unavoidable noise.