I just learned that that was apparently in early restrictor plate days, which makes sense. Vacuum = lower pressure at BDC = lower pressure at TDC, raised CR restores that to normal levels. Doesn't play nice at low RPM though.
But it does beg the question:
Does an increase in CR, with a corresponding decrease in ignition advance because pinging, result in an efficiency improvement? How about a power improvement?
Going from my 9.7:1 Barra 182 pistons & head combo to that of a 12:1 Barra EcoLPI apparently should yield a 5.4% increase in efficiency
The Otto Cycle: Compression Ratio vs. Efficiency
But if it costs power at high loads because of ignition timing being too far from MBT I'm not really interested