Why does some popcorn kernels varies so much between 100g un-popped kernels ?
Found one brand with 320 kcal per 100 and a different one with 360kcal per 100, yet they are both just popcorn kernels.
That difference is pretty much well within any expected margin of error; the way nutrition labels are calculated aren’t as precise as you might expect.
One company may have used an acceptable standardized food database, another might have had their product specifically tested. As others have noted, maybe they are slightly different varieties of corn, or water content.
Fun fact: when growing your own popcorn at home, the water content is actually really important for getting good “pop” and there are ways to measure it and then dry or rehydrate to get the desired water content. Relevant to your question because the moisture will make a tiny difference in the nutritional info - it contributes to weight when you measure out that 100g, but doesn’t add any calories. You could purchase one product that is 320 kcal/100 g at the time of packaging, but if it sits on your shelf for months and some of the water evaporates, maybe it’s 350 kcal/100 g by the time you prepare it. But it’s only an estimate in the first place so absolutely not worth stressing about!