There are other factors.
Back in the day, you could set up a wooden gantry and hit oil not too far down. I imagine the total number of wells was huge, and the productive percentage a huge number.
If you're drilling very deep wells, or offshore or other exotic wells, drilling one well is a very laborious process. Maybe you get ten wells in a year instead of 500, and the productive percentage is a much smaller actual number of wells.
Just speculating, of course. Maybe I can find the article.