Possible explanation - the metal detectors do not do a good as good a job of finding metal that is "shielded" by non metalic stuff. A thick coat, a thick wallet, or your hand in your pocket can make it harder for the machine to find something. This should come as no surprise. Again, the problem is that there are no standards at the airports that say that ALL overcoats be put on the belts. Some airports in Europe require this.