Definitely a combination of user error, material quality and groundbreaking technology.
For what it's worth, since I started to try to build ABOVE the lower limit of TP resistance, even minor resistance drifts aren't an issue anymore. Its when you operate on the borderline when things can go iffy...
Tony
I know I mentioned it before... I think another advantage of the SX350J over the DNA40 is its low resistance limit of .05 Ohms... Not suggesting that .05 should be your target resistance, but it gives a lot more headroom for fluctuation compared to the dna. If your coil dips below .1 Ohm that's a no go on the DNA40.