pool mining – no rewards for 45 to 90 min every now & then? how & why?

I am mining with the top popular pool (you can guess it easily) and notice that there are long periods of 45 min to 90 mins every now & then without any rewards. My rig is continously online with regular shares (to rule out the possibilty of not being online/not mining) from the logs.

I dont know if it is a coincidence, but when I am refreshing the miner stats (of my wallet) periodically, I do not see such behavior. But when I sleep at night (and obviously do not refresh the stats), I notice such long durations with no reward inbetween.

I understand the luck part, but I am not able to understand how can there be such long periods without any rewards intermittently.

Do you people also observe such gaps in rewards? Is there any meaningful / practical explanation for it?