Post-operative surgical pain is common after microdiscectomy. And as you have noticed, there can be a wide range of differences in pain sensitivity. Scientists are still trying to understand the complexities of pain and pain control.
The use of pain-controlled analgesia (PCA) is not new. Studies first showed that small amounts of opioids given intravenously were much more effective for postoperative pain control than standard oral (pill) dosing. And the pain relief is faster with fewer side effects (nausea, vomiting, difficulty breathing).
Individuals do have different pain tolerances. They also have different responses to the same drug. The same patient can respond to a single drug differently on one day compared to another. Women are affected differently based on changing hormone levels.
Even ethnic groups have different drug tolerances. For example, Chinese people metabolize drugs differently than Caucasians. They often need lower doses of the same pain reliever.
At the same time, surgical technique can make a difference. Minimally invasive surgery can be done with less trauma to the nearby muscles. A skillful surgeon handles all tissue carefully with minimal disturbance. The type of incision, length of time in surgery, and type of surgery are all important factors as well.