WFP mentioned the one thing that will affect fit more than any small manufacturing variation, and that is head shape. People with the exact same head measurements can experience totally different feel with the same helmet, due to pressure points created by head shape differences.
With the exception of Arai, which actually addresses this issue with different head shape models, most manufacturer's try to come up with a compromise shape that will supposedly fit the "average" head. In reality, that works about as well as all the other "one shape or size fits all" products.
The advice to actually try various sizes on prior to ordering is right on the money. And if you already have a helmet that has pressure points that bother you, check to see if the manufacturer offers different thickness pads to tailor the fit. Many of the higher priced helmets offer at least different cheek pads. And if all else fails, you can pull back the padding and modify the actual energy absorbing foam slightly to eliminate minor tight spots.
Jerry