Hyperthreading adds core count to windows task manager. TM simply divides the cpu usage across all cores by how many cores there are. Sometimes windows loads full cores before HT cores so the HT cores are 0% usage.
Calculations: cpu 80%, HT core 0%. add together, 80%, divide by threads (2) = 40%
average cpu usage, 40% because windows isn't using the HT core, but is calculating it.