Firstly, because you cannot adjust timing on the fly. If you loosen the IP while the engine is running it will destroy it, possibly taking your hand off in the process.
Secondly you can set it where it sounds and drives really nice. Butt dyno'ed better than recommended timing. And be far enough advanced that you're burning up your GP's and risking cracking pistons.
Thirdly, the collective has alot of testing, and dyno time behind their 9.5* btdc on pulse method #
Really unless you've got it on a dyno, and meter it between runs to know where you are. Ear timing, or bumping it by feel, will get you nowhere.
To the OP, keep in mind the timing curve on these pumps is a calibration. So the curve will not always be the same from idle to 2k on different pumps.