Once again, the smaller the particle (dust vs sand) the hotter it gets as it passes through the combustor.
The larger the particle (sand and young eruption volcanic stones that haven't fallen to the ground yet) the more likely it is to be centrifuged out the fan duct.
Volcanic ash particles often are made up of stuff that melts at temperatures lower than most earth surface dust-sand storms.
It's a statistical thing and you manage your way around it finding air clean enough to fly in.