Just an idea but...
Calorie king gets its nutritional information from non-branded foods (e.g., "popcorn") from the USDA's nutritional database. It gets nutritional info for branded foods from the manufacturers.
This creates two disparities:
1) Manufacturers are allowed to round down, whereas the USDA doesn't.
In this case, air-popped popcorn contains (per cup) 0.36 g. fat. Smart Pop is rounded down to 0g fat.
It's not just the fat, though: smart-pop has .5 g. protein vs. 1.04 for air-popped popcorn (not that this is a good thing!) and 5 g. carb instead of 6.23. So, here are two other hypotheses:
2) Post-farm manufacturing processes may remove more of the kernel.
3) "Independent testing" more often than not works in the manufacturer's favor. (Remember the frozen yogurt episode from Seinfeld?)
Anyway, for one-ingredient foods I always use the USDA database, because I think it's more accurate. You can access it on line directly here:
http://www.nal.usda.gov/fnic/foodcomp/search/
Kim