Because the technology has improved considerably in the last 2 years...
1.....Better control and results from directional drilling...
2....with each well drilled, the understanding of the geology improves
and the bonus point:
3....with new knowledge obtained in 1 and 2 above,....older wells are also more productive.
So you get more oil from less wells than a few years ago.
I don’t agree the number of new wells and particularly drilled well footage is significantly decreasing in North Dakota.
You might want to look at the latest state report tracking activity.
https://www.dmr.nd.gov/oilgas/presentations/NDOGCPC091013.pdf