Because the air is much more dense when on boost, the flame travels much faster through it. The retarded timing (compared to NA timing) ends up just being the right amount of timing. Try not to think of it as a late firing event. It just takes less time to make complete combustion so you don’t need to start it as soon.
Ok thanks for the clarification. I was picturing less advance timing so combustion would be after TDC and direct push down. Less cylinder pressure than a high compression NA engine. Maybe I was wrong, or just using the wrong words. Likely both.