Discrepancy in estimate of reserves

Meo has been spruiking the 8+Billion barrels figure since the start of this play but I noticed that Proactive investors quoted the US Geo Survey est. as being 4+ for the whole region?! Either Meo has picked a sleeper that had been overlooked, the US Survey is out of date or under done due to no access for a while or it is being spruiked up a tad??

I would be interested in any knowledge or reflections on this discrepancy.

From Meo press Release 7th July....
"The first of three identified oil plays on Block 9 has been assessed by MEO to contain 8.183 billion barrels of Oil-in-Place with ..."

From ProActive Investors Australia 19th August ( reporting the cash raise)
"Critically, the United States Geological Survey estimates that the total undiscovered, technically recoverable oil in the North Cuba basin, where Block 9 is located, is 4.6 billion barrels.