Original Task Manager creator explains why it lies to you about CPU usage — former Microsoft engineer shows unique solution to a seemingly simple, but actually complicated, task

Former Microsoft engineer Dave Plummer said the original Task Manager computes CPU usage from changes in per-process kernel and user time between refreshes, not from a live instantaneous reading.

That accounting-style method is more precise than a simple elapsed-time estimate, but modern CPU features can make it seem misleading.

Turbo boost, thermal throttling, and power-state changes can cause the reported percentage to diverge from how busy the system feels.

The takeaway is that Task Manager shows an averaged CPU accounting over time, and simple utilization metrics are less reliable on modern hardware.