Good question. My old technique was to just rub the spices on the meat, toss it on and let it go. Pretty simple and not much fuss. When it reached 195 I'd pull it and eat.
This time though (after trying one new thing at a time over several cooks) I trimmed the fat cap off. I figured this would give me more surface area for the bark, which it did. I couldn't see wasting rub on a piece of fat I wasn't going to eat anyway. I then injected the butt which I never used to do. I also never used to do the mustard layer either. This really helps the rub to stick and creates a nice bark. Lastly, I never sprayed with the cider/vinegar glaze.
All those components together had created a synergistic effect to produce the ultimate butt.
The last thing is my cooker was 260f the whole day with not so much as a hiccup (until the lid was opened for spritzing). I think the tempersture stability and the efficiency of the cooker played a huge role too.
Joe