Dgc has it pretty well pegged. His example of the advance needed for magnums comapared to open chamber heads is mainly port design and velocity IMO, as the factory setup was not very good at getting quench tight and consistent. On my 302's I found I needed even less timing, in the range of 28 to 30* maximum, based an exhaust temp and fuel consumption off the efi logs. There is also very little difference in the advance needed for cruise or power, which would be an indicator of very good charge mix and turbulence.

I have had the scanner on a couple of later model cars (not high perf) and found they also run very low advance compared to what we are used to. My Escort was at only 25* at 60 mph (2300 rpm) cruise.