If food manufacturers are allowed up to 20% difference in their calorie counts, then it makes more sense to look carefully at the weight of a serving. I don't think that economically speaking, the manufacturers would want to let more food go into a package than what they say, but maybe they get it all worked out in the accounting books. I'm guessing that frozen meals and bagged veggies (examples) are mainly frozen and packaged with machines doing the work, and the machines get calibrated.
A bag of potatoes is another story. What they call a serving may be hard to randomly pull out of a bag of different sized items... same with fruit or bagged onions.
If you weigh a raw potato and then bake it in the microwave and it ends up being heavier, I have no idea how that could happen. I imagine your scale must be accurate and you zero in before weighing each time so it's a mystery where those 10 calories came from!