You're point about testing methods is a great one, and I was going to make that point myself. I work as a chemist refining the next generation of testing methods, and when we use a new method we need to do large numbers of cohort studies, to document the normal ranges with the new methodology. A new methodology may make the averages go down, and it doesn't mean that everyone magically got healthier, it just means this new test is different.

I do have to be my naturally pedantic self and point out that there are not two kind of cholesterol. What you are referring to are actually called lipoproteins, for the life of my I don't know why we call them cholesterol but it's of course not just you. Cholesterol levels are an entirely different test, and there is only a single kind of cholesterol, called cholesterol. Lipoproteins are responsible for collecting, depositing, and moving around all of your lipids (triacylglycerides and cholesterol) throughout your body. HDL lipoprotein tend to play a role in the deposition of fats, especially arterial cholesterol, (which is why we call them "bad") while LDL lipoproteins tend to help to remove such deposits (why they are the "good" cholesterol). Importantly though, neither is actually a kind of cholesterol, of which there is only one (okay one natural cholesterol, I actually used to work in a synthesis lab where we specialized in making unusual forms of cholesterol).