This is the FlexibleWorkGang constructor:

  FlexibleWorkGang(const char* name, uint workers,
                   bool are_GC_task_threads,
                   bool are_ConcurrentGC_threads) :
    WorkGang(name, workers, are_GC_task_threads, are_ConcurrentGC_threads),
    _active_workers(UseDynamicNumberOfGCThreads ? 1U : ParallelGCThreads) {}

and the WorkGang constructor:
WorkGang::WorkGang(const char* name,
                   uint workers,
                   bool are_GC_task_threads,
                   bool are_ConcurrentGC_threads) :
  AbstractWorkGang(name, are_GC_task_threads, are_ConcurrentGC_threads) {
  _total_workers = workers;
}

If 'workers' are less than ParallelGCThreads, then _active_workers will be set to a value that is greater than _total_workers (unless we run with UseDynamicNumberOfGCThreads). Both G1 and CMS typically sets the number of concurrent workers to a value below ParallelGCThreads.

This is currently a benign bug, since the GCs will later calculate a new value for _active_workers that will never be higher than _total_workers.

However, this bug impedes stricter asserts and should be fixed.