In some ways it would make sense that OPEC countries would do just sufficient exploration and research each year to ensure that they could make up for the oil that is pumped. So, if they chose, they could spend more each year on exploration and technological upgrades to upgrade their proved reserves, but they know that doing so would only cause their oil to run out faster at a lower price.
On top of this, they may very well have a significant buffer of "proven-to-themselves" reserves, but choose to report slightly less than this, which allows them to maintain a steady "proven-to-the-outside-world" level.
Is there actually any "official" explanation from OPEC countries as to how they have managed to keep their "proven" reserves remaining steady for so long?

Is there actually any "official" explanation from OPEC countries as to how they have managed to keep their "proven" reserves remaining steady for so long?

Not that I'm aware of. But I see that as less than 50% of the problem. The core issue is the way that OECD institutions reproduce the OPEC statistics without question. Those inflated reserves numbers are then quoted by political leaders from around the world and provide an excuse for inaction.

We are 100% certain that this flat line reserves reporting is wrong - and it is pretty unacceptable to have a Global energy strategy that is based on numbers which are known with 100% certainty to be false (and that is not pre-judging if they are too low or too high).

In some ways it would make sense that OPEC countries would do just sufficient exploration and research each year to ensure that they could make up for the oil that is pumped.

It might make sense but it would be an incredible trick to just discover enough oil, or to just tweak technology enough, to replace production.

Not if they maintained a buffer that they weren't telling us about.