Not quite; its used to highlight any inconsistencies in testing, if the P2 is wildly differing from double the P1 number. If so, other factors could be attributing to nutrient lock or availability such as pH or soil texture. As an inhouse tool, this would alert the tester to possibly run the test again.

You can think of the soil as having two main pools of acidity, active acidity (H+ in solution) and exchangeable acidity (H+ adsorbed to soil particles). In acidic soils with a high CEC the size of the exchangeable H+ pool will be MUCH larger than what is actually in solution (active H+). Since these two pools are in equilibrium (or trying to be), active acidity is 'buffered' by exchangeable acidity. In order to effectively raise soil-pH, both pools have to be neutralized. The size of the exchangeable H+ pool can not be measured by a normal pH measurement (requires titration). This is why soil labs also report a buffer index for soil-pH. The pH measurement is basically just provides a yes/no answer to the question "Do I need to to add CaCO3?" If that answer is yes (soil-pH is too low), then the buffer index answers the question "How much CaCO3 needs to be added?" I'm not terribly familiar with volcanic soils, but with that much OM I imagine your soil's CEC is quite high. With volcanic parent material there is also (probably) quite a bit of aluminum, which would provide a natural source of acidity through hydrolysis. The combination of the two would give a soil with low pH and high CEC, which basically means its going to take a lot of lime to raise soil-pH.

4tons/ha is not that much