The logging of number of workers used by the young GCs in G1 is wrong.

The code looks like this:
---
    uint active_workers = AdaptiveSizePolicy::calc_active_workers(workers()->total_workers(),
                                                                  workers()->active_workers(),
                                                                  Threads::number_of_non_daemon_threads());
    workers()->update_active_workers(active_workers);
    log_info(gc,task)("Using %u workers of %u for evacuation", active_workers, workers()->total_workers());
---

This looks ok, but if anything goes wrong in when updating the number of active workers the value might not be what is expected. This is easily triggered by using -XX:+UseDynamicNumberOfGCThreads -XX:+UnlockDiagnosticVMOptions -XX:+InjectGCWorkerCreationFailure.

The fix is also simple, just print workers()->active_workers().