What happened to the 318 in 1971?

It means you take the factory CR's with a BIG grain of salt LOL. BTW, the combustion chamber volumes listed above are waaaay low anyway so that explains your results being + and -; if you go with real numbers, the actual CR's are all well - from those listed. For example, no way a standard production 675 head chamber is going to be 60 cc's..... I measured a stock set at 67-68 cc's. The cc's listed are all some sort of production dream numbers LOL.