There were several additional reasons, but it seems from the report that the USGS did not actually consider any of those additional reasons, based on the results (substantial increase in number of wells, longer well prouction histories, new technology, increased cooperation from industry experts, etc).
The "new" 2013 number for the Bakken Formation: 2.81 to 4.61 (mean: 3.65 billion bbls).
The "old" 2008 assessment for the Bakken Formation: 3.0 to 4.3 (mean: 3.65 billion bbls).
Although the mean remained the same, the "F95" for the Bakken actually appears to have decreased. (This is based on the narrative for 2008 and the slide presentation for 2013, but those are the numbers provided.)
That's the observation. It's hard to believe that five years of new data, new technology, more wells, increased fracking stages/well, the assessment did not result in any change in the mean, either up or down. There are four possibilities for the Bakken mean not to have changed over five years:
a) the Bakken Formation was not reassessed; only the Three Forks
b) the oil industry has not gotten any better at recovering oil from the Bakken Formation
c) the oil industry has gotten better but the USGS took that into consideration back in 2008, and thus no change in the assessment
d) the USGS is really, really good at making estimates, and five years later, their assessments have not changed for the Bakken Formation