When OPEC sets production targets they all haggle with each other over what they'll each produce. This is based on things like percentages of reserves, percentages of capacity, percentages of historical rates, percentages of historical revenues, etc. Over the course of decades and dozens of iterations of negotiations the underlying numbers have become, lets just say, very very loosely coupled with reality. They are mostly produced by literal monarchies (I'll never get over this) or military dictatorships with no third party verification. So in addition to being part of an economic/revenue negotiation, they are also part of a regime-stability bluff.

That is why, as a general rule, every year they "discover" approximately the same amount of oil they produced/delivered that year.