真实的qps有时和FlowRule中设置的qps的不一致

我将wiki中的案例代码拷贝到我的IDEA中，然后稍作修改，具体如下：

public static void main(String[] args) { String resourceName = "helloWorld"; initFlowRules(resourceName);

    long begin = System.currentTimeMillis();
    long end = begin + 1000;
    int success = 0;
    int fail = 0;

    while (System.currentTimeMillis() < end) {
        Entry entry = null;
        try {
            entry = SphU.entry(resourceName);
            success++;
        } catch (BlockException e1) {
            fail++;
        } finally {
            if (entry != null) {
                entry.exit();
            }
        }
    }

    System.out.println("Begin time: " + begin);
    System.out.println("End time: " + end);
    System.out.println("Success: " + success);
    System.out.println("Fail: " + fail);
    System.exit(0);
}

private static void initFlowRules(String name){
    List<FlowRule> rules = new ArrayList<>();
    FlowRule rule = new FlowRule();
    rule.setResource(name);
    rule.setGrade(RuleConstant.FLOW_GRADE_QPS);
    // Set limit QPS to 20.
    rule.setCount(20);
    rules.add(rule);
    FlowRuleManager.loadRules(rules);
}

初始化FlowRule时，设置的qps为20，我期待程序运行结束后，success输出的值也应该为20， 然而实际运行过程中，日志显示success的值 有时为20，有时为40.

Begin time: 1533132761258 End time: 1533132762258 Success: 40 Fail: 140633

原提问者GitHub用户xflonga