That is not exactly what the article is saying, and I doubt that the people running Nigeria actually really knows how much oil the country produces. For one, the people administering NNPC and other government agencies have not been the same since oil exploitation began.

Apart from that, from the article, it seems that the oil companies are the ones that report how much they produce to NNPC, which leaves open possibilities for manipulating the figures. Actually, the article mentions 'about $243 million in the areas of under-calculations in terms of royalties; about $340 billion in terms of petroleum profit tax.'

Subsidies are also another component of the calculation that is very problematic. I actually doubt that anybody knows actually how much the country spends on subsidising the importation and consumption of petroleum products in the country.

Add to that the fact that there is illegal bunkering going on in the Niger Delta region, which means that a great deal of oil is sold illegally on the seas.

For more, see my earlier posts here and here.