I do not know HOW it stays, except the wad beneath it, is pushing on it. It certainly seems to stay just from the accuracy achieved and clean shooting with black powder loads. Without it, the fouling tends to build up, which hurts accuracy.
Middle row. These are 16 bore cases - 14 bore wads (oversize) with 14 bore and 16 bore balls for the rifled 13 bore barrel. The cloth patched 16 bore balls were the most accurate, and of course, the cleanest shooting.