This occurred to me last night when I was working up a 1965-1970 full size Pontiac for hooning around in BeamNG. The compression ratios might not be as inaccurate as you think due to Automationeers’ tendency to use very conservative and unrealistic fuel system and in particular carburetor tuning.
Not to boast, but I made a rather lengthy post about this in my American car realism thread. Basically, a carburetor with an AFR leaner than about 13.7:1-ish is pretty much never seen in road car applications. I found when I was building a faithful Pontiac 400 V8 that if you push the carburetor tuning into its realistic territory (I was using 13.0:1 on the high compression 325 hp 400 4-barrel), the disparity is a lot less or even eliminated. I managed 9.7:1 compression on my 325 hp 400 4-barrel which is reasonably close to the 10.2:1 compression of its real life counterpart.
Ignition timings on stock engines are also similarly not quite as people probably think. At idle, many engines will fire at or only a few degrees before top-dead-center (TDC). Comparatively, many Automation players have learned that more advanced ignition timing is virtually always better so I would guess many players’ engines are firing more like 10-20 degrees before TDC at idle.
Combine those two things and I would wager that compression disparity is a byproduct of min-maxing strategies in a video game rather than anything real.