Why doesn't pain always match the injury?
Because pain is produced by the brain, not delivered by the body — and the brain is weighing a great deal more than tissue damage when it decides how much to produce.
The old model was wrong. For centuries pain was treated as a signal travelling from an injury to the brain, with intensity proportional to damage. That model cannot explain the ordinary observations: soldiers with severe wounds reporting little pain; people with a minor cut in the wrong circumstances reporting a great deal; chronic pain persisting years after tissues healed; phantom limb pain in a limb that does not exist.
The current understanding. Nociception — the detection of potentially damaging stimuli by nerve endings — is not the same as pain. Nociceptive signals travel to the spinal cord and brain, where they are modulated at multiple points and integrated with other information. Pain is the output: the brain's assessment of how much danger the body is in and how urgently you should act.
What that assessment incorporates: attention and distraction; context and threat; expectation and belief about what the sensation means; previous experience; mood, anxiety and sleep; and social factors.
This is not "pain is in your head" in the dismissive sense. The pain is entirely real; it is the mechanism that is being described.
Why chronic pain is different. In persistent pain the nervous system itself changes — central sensitisation, where the spinal cord and brain become more responsive, amplifying signals and sometimes producing pain from ordinarily harmless input such as light touch (allodynia). At that point the pain has become the condition rather than a report about tissue, which is why treatments aimed only at the original site frequently fail.
The descending pathways from the brain can suppress or amplify incoming signals, which explains both stress-induced analgesia and why anxiety genuinely makes pain worse.
Why this matters: explaining these mechanisms to people in chronic pain is itself an evidence-supported part of treatment. Persistent or unexplained pain should always be assessed by a clinician.