Measure how far the pistons are in the hole. A 10:1 factory engine usually is actually closer to 9:1. With the large combustion chambers on the chrysler big block heads its hard to get a true 10:1 in a 383 without pop up pistons.
Yes, the 340s were like that too. Rated at 10.5:1 but more like 9.5:1 actual. The factory compression ratings were not accurate. Thickness of head gaskets used can also make compression higher or lower.