Peter, You need to read that more carefully the test/study its referring to rendered 40 DCS (decompression sickness) incidents (5%), of which 21 were moderate DCS, 18 mild DCS, and 1 serious DCS.
You have misread the 2000 to 8000 figures.
The rule is 18 hours and defined as 1000 feet! Its a test question on the Exam and printed on the recreational Dive Tables every beginner diver is given.

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