What happened to the 318 in 1971?

I was looking over my old Motors Manuals yesterday and while looking the specs of the 318 I noted that for 1970 and prior years the cylinder compression was 140 psi and compression ratio was either 9.2 or 8.8. Beginning in 1971 the compression ratio dropped to 8.6 and compression was listed at 100#. That is some kinda major drop! What, exactly, made this change?
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