Actually you were spot on for some of the causes for the discrepancy, but his readings aren't uncommon. You also have the factory machining tolerance for the rod length to factor in. It is +/- .005", so that in itself could account for the variance. Also the stroke of the crank has a +/- tolerance of a few thousandths, not to mention if it had ever been reground before and was off a little by whoever ground it. So, a lot of little factors adding up to still be in spec by GM's guidelines, but not so much when you try to tweak the engine a little better for performance purposes.
