WHY IT LOST JOBS?

I just have run a spider program and the first script will add many jobs into the queue, then the other two same scripts will get job from the same queue continually. However, when I check the QSTAT of the queue ,it shows job-in not equal to job-out since the QLEN is 0. Why / How this situation come out? It may be dangerous. And later queue info is nil since the queue length is 0. Here is the infomation I copy from disque-cli: