This question cannot be answered clearly because the methods used by those organizations to estimate oil and gas resource volumes are often not publicly available to compare step-by-step.
One common reason different organizations get different results is based on what the assessments include. USGS estimates are for undiscovered, technically recoverable oil and gas resources. The USGS does not include discovered reserves in its estimates. The USGS also does not include reserves that have already been produced. Finally, the USGS does not include resources that might become technologically recoverable in the future but are not presently recoverable.
One advantage of USGS oil and gas assessments is that they are based on a publicly available methodology that is used for all oil and gas assessments. This methodology makes it so that our assumptions and methods can be seen and examined, and the fact that we use the methodology for all of our assessments means each assessment can be compared to the others in an apples-to-apples comparison.
Learn more: USGS Energy Assessments