Pertronix install vs. wiring diagram

Its not for trying, but I just can't seem to wrap my head around this "change of phase".

In reading back trhough the installation instructions, at no point do they suggest removing the distributor.
1. Remove distributor cap and rotor from distributor...
and the last step
21. The engine can now be started. (Which it did, easily.) Let the engine run for a few minutes and then set timing in the conventional manner.

They way I read this, it was a given that the timing was going to be wrong, or at least different, after installation. The approx 20' difference could be no more than the relationship between where the pick-up is located on the breaker plate and where the 8 magnets are located on the reluctor, which is keyed to the distributor shaft cam. Because of that, it seems to me the maximum the timing could be off would be 22.5'. (Half of 360'/8.)

Maybe I'm being too simple, but I'm unable to read more into it than that.

Mike in FL