when companies make extract, they are basically doing the mashing/sparging process for you. you really have no idea of the grains, quality of grains, or temperature(s) they are using. if they mashed at a lower temperature, then you will have more fermentables and can end up under 020 (sometimes well under). if they mashed at a higher temperature, then there will be more unfermentables (carbs too big for the yeast to eat) in the extract--just as mpavlik22 said. in extract brewing, we give that control to the extract maker.
another thing that could easily influence your fermentability is the use of specialty grains. depending on the process, specialty grains can add unfermentables, which would raise your fg. it just takes time and patience.
i have experienced this many times. most of my extract brews finish at 020. i have had better success with some extracts, but the general fg is usually in the 020 range.
with your iipa, what was your og? if you started in the 1.075 to 1.080 range, then your abv will be around 7.22% to 7.88%. that's not too high, but it is inching into the iipa range.
just keep sanitizing, growing/pitching enough yeast, treating the yeast right, controlling your ferment temps, and, of course, rdwhahb.
