Where do the sparks that the spark arrestors arrest come from?

[Nitroman]

I have seen sparks fly off the chain on my 394 while cutting dry, perfectly clean logs. I think bar oil is cheaper than a chain and bar so I keep mine maxed out. I started a fire with the same 394 cutting a major log when the exhaust caught the bark on fire. I also smoked up a stump with my 3120.