Well, I'm NOT Pat, but it's really simple.

A quality machine shop will mic each piston and bore a given cylinder for that particular piston. Generally this is done only on race engines where tolerances can be critical.

However, manufacturers do vary slightly in setups when they machine parts (any part, not just pistons) and a good machine shop will want to know what they're dealing with before they start "whittling away" on a block.

Hope that helps.