But in the end that's still not actually true in many modern desktop chips. You can take a 65W part, and with a "stock" motherboard firmware, good cooling, and the right workload end up averaging way more than 65W. Or if you have it in a hot room it just might end up using less than 65W.

TDP is more of a rough idea of how much power the manufacturer wanted to classify the part as. It ultimately only loosely relates to the actual heat or electrical usage in practice.