+1. Type of grain is another variable. As far as an all-grain brewer goes (starting from the malted grain (and sometimes unmalted), and not the malt extract made by a company), all the variables are adjustable. Type of grain, mash temp, whether you use nearly 100% fermentable sugars like cane or corn sugar, yeast strain, amount of yeast pitched, ... These variables all determine what the FG will be.
With extract it's a little more limited to the brewer, but not entirely. Basically there are just more unknowns. If it's malt extract, whether it's dry (DME) or liquid (LME), it is made of grain originally as well. But it's hard to know what type of grain the malt company used to make the DME or LME; for instance, how much crystal malts were used to make an "amber" DME? Also, what was the mash temp they used? and (i'm assuming this matters to how well malt extract ferments) how did the company dry the "wort" so it was concentrated dry or liquid extract (i.e. whether there was caramelization that made some of the sugars that were fermentable into unfermentable)? Also, how old is the extract? Has it degraded on the shelf and might not be as fermentable as it was when it was fresh?
Regardless, I think it is safe to say that beer brewers don't (generally) stop fermentation purposely. It can all be (if all goes well) determined by ingredients and process.