'For' duration of Alert Rule is not working?

Hello All:
According to grafana doc:Create alerts | Grafana Labs


Pending Alert will keep up status ‘For’ duration.But I have a alert from Pending to Alerting not keep up For duration. For example:

‘For’ field is 5 minutes,but alert from Pending to Alerting just spend one minute.
Why?

I think you should increase the second parameter of the query query(A, now-10m, now) to count with more data.

Could you try?