I find this rather perplexing too, because as an avid calorie counter for years (I'm a little overboard on it lately) I can't find a good statistic on popcorn. I've always been confused with the popped/not popped discrepancy.
Maybe it has something to do with how big the kernels pop...not all popcorn is created equally
I would go with how many calories are in unpopped kernels because they are easier to measure, and if you popped 1/2 cup I think it's 280 calories. I also double checked that amount against a couple others and it matches up a lot better (though they say 300 calories, not 280)