I often come out from my short trips with an entire day's worth of food left over - but there's a method to my madness. Here in Southwest Ohio, we have small pockets of near-backcountry; I live within 20 minutes of such a 35-mile trail. I often go out for a quick overnight hike, but I always carry two full days of food - because every so often, I'll get up and start hiking the second day only to realize I don't really want to go home yet. Since it only takes about 15 minutes on a cell phone to extend my camping permit for an additional day, I always carry that second day's food, just in case. (I don't really think that's what you meant by "leftover=poor planning" but I couldn't resist. )