With respect to "stability" - its not just about "keeping the numbers that we can measure constant" because keep in mind that the stuff we are measuring - alk/ca/mg/no3/po4 is only the amounts that are left in the water. There's a lot of stuff happening in your tank that we cannot measure and those measurable numbers is only the tip of the iceburg that gives us an idea of what's happening in your tank, but not what's actually happening in your tank.
As a simple example - your testing shows that no3 is "stable" at 10PPM for 3 months in a roll. During this time you kept your feeding constant, for ex, at 50 units of "N" per month, and you kept your nutrient export constant, for ex, at 15 units of N per month. That "should" mean that your tank uptakes 25N (50 - 15 - 10) a month and is stable right? Not quite - in these 3 months one or more of the below could be happening:
1) Corals uptake 15N, pods take up 5N, algae take up 5N
2) algae bloom - takes up 20N, coral takes up 5N, pods starve
3) pods explode after algae - takes up 10N, new algae takes up the remaining 15N, Corals starve.
In the tank, these three sources constantly jostle for dominance and compete for nutrients. As you can see from the example, test kits show "stable" 10PPM, but doing this time your coral is experiencing nothing stable. It takes a long time for these three source to reach an equilibrium - and this is just one of MANY biological processes that's happening in your tank - others include Phos cycle, Carbon cycle, Calc cycle - all these have their own checks and balances where measuring po4/alk/calc doesn't give you the full picture. Plus, every time you dose a medicine/chemical treatment, add a filtration gadget, have some kind of "outbreak", do a large water change, it tips the balance a bit and can make the maturation process longer. This is why we say a tank takes a LONGGG time to mature - up to 1 year or longer, and wait before adding SPS.
I would also caution the "other corals have looked good for 3 months". While that is a very good sign and show that your tank is on the right path, please keep these two points in mind:
1) different species have different requirements.
2) Corals, like any organism, has certain degree of adaptability, so can look "happy" on the outside for months or years in suboptimal environment but the inside slowly weakens and its life span reduces until a certain minor stressor tips the scale over and it dies seemingly "for no reason". there's a lot of stories of "corals doing well for 8 months then suddenly die", a lot of that is due to this long term accumulation of negative factors that have very little outside sign.
