Well, the problem is in the yield and how you eat popcorn.
For example (I pulled this info online
1/4c unpopped kernels = 131 calories
1/4c unpopped kernels = 4-5 cups of popped corn
1 cup of air-popped corn = 31 calories
31x4.5 (to average) = 139 calories
Since I'm pulling this info off the web, it's not going to be quite the same as pulling it off of a bag with both labels. You can see though, that popped versus unpopped is approx the same calorie wise if you are eating the same amount of kernels. So, if you pop 1/4c of unpopped kernels and eat it all then count by the unpopped number. If you don't intend to eat it all, measure it out once popped and go by that number.
I hope this all made sense!