You can only count it as part of your reserves if you have high likelihood of recovering it with current technology. Technology has increased greatly since 1995. Back in 1995 the oil was unrecoverable therefore was not counted as part of the US reserves. Not surprising at all.
there is also the cost of extracting and refining it. if because of the type of rock it is in, or if the type of oil that comes out of the ground requires extra processing, it costs $200/barrel to produce, its not going to get to market any time soon.
also this article states the Bakken Formation contains 3.65 billion barrels. Saudi Arabia contains 258 billion, Kuwait 92 billion, Iran 93 billion, Iraq 100 billion, UAE 92 billion.