"Fat: 0g +/- 20%?"
Although this doesn't automatically mean everything you eat has 20 percent more calories than it says on the packaging, we have an inkling which side most companies err on. If these lax standards bother you, perhaps you'll be comforted by the fact that it doesn't matter. There's no process in place for regularly auditing food nutrition labels, so it could all easily be lies.
Yes, using the honor system to police an industry that has repeatedly shown a serious deficit on the honesty front has gone roughly as well as you'd assume. In 2008, the Government Accountability Office researched how often manufacturers' vitamin and mineral claims fell within the FDA's 20 percent rule. Of the 300 products tested, 47 percent were outside of the accepted range for vitamin A, 31 percent were outside of the accepted range for iron, and 12 percent were outside of the accepted range for vitamin C. The FDA allowed five different methods to determine a food product's calories, all of which yielded wildly different results. If you ran one of the shadier health food companies and were about to launch a new product, wouldn't you use whichever method looked best on paper, regardless of accuracy? Of course not, because you have integrity. Which is why you don't run a shady health food company.