Turbulence within the combustion chamber accounts for much of the static advance above a certain RPM. That turbulence will cause the flame to complete its travel from the spark plug to the far corners of the chamber in less time as RPM increases. Combustion chamber shape determines much of the turbulence. Not sure I've worded that correctly but some engines run fixed advance.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.