There can be. If you have a month with cancellations for weather, you are still pay protected for the time you didn't fly...assuming you're a line holder. You also have the greater of "scheduled block or actual flight time" that can sometimes add a couple hours of credit over what you actually blocked...if you also underblocked some legs.

For reserve pilots it's completely different. You'll get paid 75 hour guarantee, unless you exceed 75 hours of block time...which I believe is pretty rare.