Why do the fibre wads in some cartridges disintegrate into lots of little pieces, and others don?t?
Do you mean that ‘falling confetti’ effect you sometimes get when you fire a fibre-wadded cartridge?
If so, then I once got curious, so fired one into a straw bale at short range, and dug out the wad.
A layer (or maybe several layers) of thin, shiny paper on the back of the wad had been burnt off, but otherwise the wad was intact.
If a wad does completely break up outside the barrel, so what?
It has already done its job as a gas seal between the powder and the shot and ? arguably ? if it does break up, then it should degrade more quickly.