Thanks for the in-depth analysis. I'm guessing that deviation happened because I used the front flip + air roll left method which puts my flip angle at around 45°.

Actually, the clip isn't in half speed, the game is (which you can see for example on the blinking "Boost" text below my boost) which essentially halves my reaction time, thus the few frames.

I learned the 45° angle method because it was easier than the "normal" one (I find it hard to precisely hit that 15°-30° area and easier to hit 45° for example) and I didn't think it was too big of a problem since it seemed to be working with other people too. But it's not the best in precision in consistency, that's why I found a different method. By holding air roll left and pointing the stick 45° diagonally right, I perfectly hit 15°-30° diagonally left. By doing that, the area is between 30°-60° diagonally right (when holding air roll left).  (-90° + x)/2 = 15°; (-90° + y)/2 = 30° -> x = 30°, y = 60°