All jokes aside, you'd be surprised how off those can be in seemingly innocuous ways sometimes. Like cereals will list a volume and a weight for their serving size, but if you actually get that amount in volume, it's like 50% heavier than it should be. On top of that, you can't assume that a package is exactly the weight that they advertise. On something more expensive like berries, they're usually pretty spot on, but it seems like most packaged foods I've weighed are around 20% heavier than the package indicates. That said, calorie counting is simple as shit, even if you're systematically off. Still gaining weight? Choose a lower calorie target and watch that weight melt off.