What's the logic behind this chain length estimation method?
Big-big and through the dérailleur is the most chain you'll ever need as you can't shift to any gear which takes more chain than this.

You will then need to add some links to take into account the extra chain required to go through the dérailleur.  2 links is the least amount you can add and when the rear mech is as stretched out as it would near maximum extension, the chain doesn't need to be much longer to accommodate this.

Theoretically you could put the chain on big-big and through the dérailleur and try and gauge how long the chain should be for the mech to be almost fully extended but not quite, but by doing big-big plus 2, you should get a similar result.

On your side note that each method gives different results, this could be because there is no correct answer.  The big-big plus two gives you a safe chain length in that if you were to shift into big-big there would be sufficient chain to do this.  If you don't cross-chain, you could get away with a shorter chain length, but risk damaging something if you were to shift into big-big.