GHC 8.2 GC time stats going negative

Hi, I am trying to use the mutator_cpu_ns and mutator_elapsed_ns from GHC 8.2 RTSStats structure retrieved using getRTSStats API documented here - https://hackage.haskell.org/package/base-4.10.0.0/docs/GHC-Stats.html . I am seeing that the values of these stats retrieved about 30 ms later are lower than the values retrieved earlier. Sometimes even after 100 ms the values decrease from the previous ones. However, as the time duration between two calls increase this becomes less and less likely. Is this an expected behavior or am I using it incorrectly? If it is expected, why does it happen, what is the underlying mechanism that makes it happen? Thanks, harendra