Fire that generates from world generation (such as patches of fire in the Nether) will never receive block updates and thus will never be able to spread. This is most obvious in the Nether.
To reproduce, place tons of flammable material around a patch of natural Nether fire. The fire will never spread to any of those flammable blocks.
However, when the player places the fire and then surrounds that fire with flammable material, the fire will spread.