I doubt if they underestimated compression, my guess is that they might have overestimated compression?
for car engines (in some years, truck 235s might have been different):
1958-63 rated at 8.5
1956-1957 rated at 8.0 (848 head introduced in 1956)
1955 rated at 7.5
1954 7.1 or 7.5 (manual or Powerglide)
