Interpreting the variability of calcium on contrast-enhanced CT

Hello, I’m attempting to calculate the Agatston Score on CT images with contrast. According to the method, calcification is considered present when HU values exceed 130. However, in my images, calcification begins at a range of 650 to 850. Additionally, the HU values of the aorta are approximately 400, but they vary between studies. How should I interpret this variability in contrast-enhanced CT scans?