That is not true. What is happening is changes in technology, such as steerable horizontal drilling, makes producing field that prior were not economic, now worth going after.
Also, old fields get additional infrastructure and technology installed. Where the original drilling program could only produce 25~30% of the oil in place, enhanced oil recover techniques such as water or CO2 flood allow more oil of existing play to be recovered. Now we get something like 60% of the oil in the field recovered.
Think of a glass of dry sand. Now pour oil over it and let it soak in. If you only stick a tube down the middle and try to draw the oil out, you only get a fraction of the oil. Now add tubes around the outside of the glass and push water down them. Eventually you start cycling the water from the outside into the center draw point. This will wash out additional oil to be recovered. No new oil is added to the glass, but you now get more out of the field.
Reserves are not total oil in place numbers. They are only the amount that can be economically produced with today's technology. If prices go higher, or technology advances, the same field can now be claimed to have higher reserves without new oil coming into the field. The only difference is the economic ability to produce more oil.
Price has a lot to do with that as well. At $90/bbl, pulling out this old oil becomes profitable. The same goes with shale oil. But at $50/bbl, these sources no longer are profitable.
The reason oil prices are so volatile is because there is such a huge diversity of marginal prices. In Saudi Arabia, they can sell oil at $10/bbl and still make money. But there is no way that producers in Alaska, California, or even North Dakota can make a profit at that price.
In fact, the Monterey is thought to hold over 400 billion barrels of oil, according to the U.S. Geological Survey. That's nearly half the conventional oil in all of Saudi Arabia.
And don't forget the Green River Formation that weighs in at 3 Trillion barrels, with 1 Trillion recoverable with todays technology
Note: 1 Trillion barrels is about equivalent to the entire worlds known oil reserves.