Because the fuel injectors are different, and they may be managed differently. You're already running the lowest fuel pressure of any Bonneville.
The stock regulator on the 94/95 L67'* is only 2.7bar. All other Series 1 cars are 3.0bar. All Series 2'* are 3.5bar.
If you want to run lean under boost and go boom, go right ahead.
There'* something funky about that one. Higher fuel pressure (in general) will give a better spray pattern and better fuel atomization.
There are 2 of us currently running 4.0bar on our adjustable regulators. (3 AFPR'* are currently running on this Forum).