why is lead assignment not triggering on the first frequency (hourly)

In our CX Sales environment, we're seeing that leads are not being assigned at the frequency increment we set. Our frequency is now set to 'hourly', so we expect (and have tested this on our testing environment) leads to be updated an hour later.

However, on our production environment, it seems to take more than the first frequency. In my test example it took three hours.:

Any idea what could be causing this?