You just have to understand that fire is HUNGRY.
Fire loves oxygen and fuel, and hates whatever doesn't have enough of it to catch immediately. Some materials are too "cool" for fire (usually because they have more water content) and some materials do not combine well with oxygen. The melty shit is determined by how quickly fire runs from place to place.
Fire is happiest when eating. When it can eat alot, it stays. When it can't, it moves on quickly.
That's how it works.