The key word is “recoverable”. If 4-5 billion barrels are recoverable, more is down there. Perhaps the earlier estimates of hundreds of billions of barrels is wildly optimistic, but there is certainly more than 4-5 billion. Recoverable oil is based on existing technologies as well as the economic feasibility. Technologies have advanced dramatically since the mid 1990s, so it would be expected that the recoverable amount would increase. Also, the price makes it more feasible. Sometimes the technology exists but it’s too expensive. Imagine that some technology existed that could extract 30 billion barrels from this field, but it cost $150 per barrel to extract it. No one would bother drilling at the present time because they’d lose a fortune. When oil was $15 per barrel in 1995, they knew there was oil down there but it wasn’t feasible to extract. Over time, expect this field’s recoverable barrels to rise and fall as the price of oil fluctuates and technologies improve.