There are also different levels of metabolization. Most people are "Normal meabolizers" and could expect all the prescribed effects of Codeines while others are "Extensive metabolizers" and should expect a quicker reaction and stronger effects. There are also some who are "Poor metabolizers" and, I would expect, have slower responses and less exaggerated symptoms.
It could just be how your body responds to codeine. I doubt that you would be a "Poor metabolizer" though because you say you do still get strong effects from codeine.