Just think of fatwood, like congealed blood. Kill some one, let him sit for a period of time, long enough for the blood to settle in the lower half of his body. It pools at the lower points and then hardens and thickens. This is how, when you kill some one and leave him in position for a while, then you try to stage his death with a pile of blow, and a bunch of hooker brochures, an experienced crime scene detective will know that the crime scene was staged, and the guy did not really die riding the blow up porpoise.
Fat wood is the same, only there is no staged crime scene with a blow up porpoise, or blow, or hooker brochures.
The sap will settle, and harden. If the tree is standing up for a long time, then falls over it won't necessarily be all at the bottom. It can settle in spots.