^With the Hornady Headspace gauge it is hard to get exact measurements.... it does good enough to get you in the correct range- but if you measure cases multiple times (without looking at the display) you will get different results. When you look at the display you can game it to get the result you want.

Even when the cases are fully formed to your chamber some will read slightly different. They expand when you fire and then contract as they very quickly cool.... and they don't always contract the exact same amount.