The duration of each round varies based on luck. Sometimes we are unlucky and it takes 4 hours to finish a block. Other times, it takes 2 minutes.

As an extreme example, let's say your hash rate is very slow, so that it takes you 3 minutes to submit one share. Well, if the pool is lucky and finds a block in 2 minutes, you wouldn't even have submitted your one share yet. Therefore, pool says you did 0 work, and so you get 0 payout. On the other hand, someone has a very fast hash rate, and so in those 2 minutes they submitted 100 shares. Then they will obviously be paid for that work.