I heard somewhere that the 'recommended portions' for dog food are often based on the needs of professional, active working dogs (ie mushers, racing dogs, etc.), because they're often the ones used in feeding trials, but I can't find a source on that at the moment so don't quote me on it. Would explain why the general consensus is that most recommendations the bags give are way too high for the average pet dog, though, if it is true. (That or the old 'they do it to get you to buy more food' explanation, not sure if I buy that one).
Either way, it's perfectly normal to have a dog need way under the recommended amount. Like people, even two dogs of the same size, build, and activity levels can have different metabolism, so it's best to use the recommendations as a (very rough) guideline, and then adjust based on body condition (and appetite, if you don't have a bottomless pit like mine, haha). I'd only worry if you start actually seeing problems with her overall health, as the others say.
