It's from SAE 770086 - The Effect of Oil and Coolant Temperatures on Diesel Engine Wear
The chart I posted is for a gasoline engine. The study shows another chart for a diesel engine that has a more gradual reduction in wear with increasing coolant temperature. Do you know where the other charts you've seen come from? Are they from diesel or gasoline engines?
Some more recent studies with gasoline engines also seem to show a pretty sharp inflection point, at a coolant temperature of 30-40°C. The inflection point seems to correspond to the point where the combustion chamber temperature rises above the dewpoint of the water vapour in the exhaust gas.