The 2,000 PSI is only for a very short period of time, the 100 PSI for a long period of time and will uncover any real leaks.

If there is a fixed size hole, 2 x pressure does not equal 2 x the flow. Think of a storm / screen door pneumatic closer. The door closes at a certain rate by it's self, push with 2x the force of the spring and it won't move 2X the speed, it will move at a somewhat slower rate than 2x speed.

There is a term for this that I can't recall right now, it is a fluid dynamics issue. Basically, a fluid or air can only move at a certain maximum speed before becoming so turbulent the fluid it's self causes a restriction.

On the engine that was using a quart every 4,000 miles, the head gasket looked perfect. There was only a very slight discolored area where coolant was making it's way into the combustion chamber. If I hadn't done a leak down test looking at the radiator level, finding where the coolant was being lost would have been impossible.

Doing a leak down narrowed things to a specific cylinder. Actually 2 cylinders, one on each side of a GM 4.3 V6. Rather than have the car down for a longer period of time, I sourced replacement heads, built them up then swapped. The head faces on both heads had a slightly low spot in the same place causing loss of coolant.