I have found the best site for temperature of Kentucky Lake is the Hancock Biological Station site, where they give both surface and bottom temp.
Hancock Biological Station - Kentucky Lake Water Quality Monitoring
I find it interesting that the site you posted and the Kentucky Lake dot COM site vary by more than a foot.
Kentucky Lake Levels, Conditions and Water Temperatures & Lake Barkley Lake Levels, Conditions and Water Temperatures
I like the site you posted because it shows many more lakes than Kentucky Lake.