Let me paint a picture. It's fictional, though a very real portrait of how things truly happen in life.
Michael is an unsuspecting 40-year old man. He hasn't undergone any testing: no heart scan, no lipids or lipoproteins. But we have x-ray vision, and we can see what's going on inside of him. (We can't, of course, but we're just pretending.) Average build, average lifestyle habits, nothing extraordinary about him. His lipids/lipoproteins at age 40:
--LDL cholesterol 150 mg/dl
--HDL cholesterol 38 mg/dl
--Triglycerides 160 mg/dl
--Small LDL 70% of all LDL
At age 40, with this panel, his heart scan score is 100. That's high for a 40-year old male.
Fast forward 10 years. Michael is now 50 years old. Michael prides himself on the fact that, over the past 10 years, he's felt fine, hasn't gained a single pound, and remains as active at 50 as he did in 40. In other words, nothing has changed except that he's 10 years older. His lipids and lipoproteins:
--LDL cholesterol 150 mg/dl
--HDL cholesterol 38 mg/dl
--Triglycerides 160 mg/dl
--Small LDL 70% of all LDL
Some of you might correctly point out that just simple aging can cause some deterioration in lipids and lipoproteins, but we're going to ignore these relatively modest issues for now.)
Lipids and lipoproteins are, therefore, unchanged. Michael's heart scan score: 1380, or an approximate 30% annual increase in score. (Since Michael didn't know about his score, he took no corrective/preventive action.)
My point: If we were to make our judgment about Michael's heart disease risk by looking at lipids or lipoproteins, they would'nt tell us where he stood with regards to heart disease risk. His lipids and lipoproteins were, in fact, the same at age 50 as they were at age 40. That's because measures of risk like this are snapshots in time.
In contrast, the heart scan score reflects the cumulative effects of life and lipids/lipoproteins up until the day you got your scan.
Which measure do you think is a better gauge of heart attack risk? I think the answer's obvious.