Undiscovered Discovery Discretization

An article on the BBC this morning has me somewhat baffled. I'll not go into a discussion about whether the Arctic should be fair game for drilling, or the can of worms that this will re-open in terms of land claims north of 66.5° latitude.

According to the survey, the Arctic holds about 13% of the world's undiscovered oil, 30% of the undiscovered natural gas, and 20% of the undiscovered natural gas liquids.

If its undiscovered, then how does one determine a percentage, especially one as precise as "13%". Since there is apparently an actual volume of oil discovered by this survey, why not use this to calculate as a percentage of the known oil reserves. Ass backwards calculations like these will go a long way to pissing people off when getting the runaround about oil prices and economic 'forecasting'.