Increased oil production

Improved extraction techniques used to exploit the oil resources that were not before. Economic conditions, including the increase in oil prices, can make the exploitation of certain deposits profitable. These two conditions, supported by a global oil consumption growing rapidly, allowing the increase in oil production. Deposits discovered in 20 or 30 years are not a lot of potential, and continue to operate essentially the old large deposits. The increase in proven oil reserves is therefore that much of probable and possible reserves become proved reserves. 

All this means that the ultimate reserves decreasing gradually and the reduction of reserves tends to s' speed.