A low-carb diet doesn't have to be intentionally low calorie. That's the point. Eating very low carb allows the body to adjust hunger levels to match existing fat stores. If you're fat and you allow your body to do its job, you won't be as hungry because you have plenty of energy stored up and the body is eager to use it. In other words, we've evolved to survive times of plenty and times of famine, and our modern diet with extremely high carb intake throws this system out of whack.
Low calorie is just forced starvation and although you will always lose weight this way, it's an uphill battle because you're fighting the body's fat regulation mechanism. You might be eating high-carb but low calorie, causing insulin spikes, extreme hunger, and protein wasting. It's not an easy diet to follow and it might even be counterproductive.
This is the theory anyway. This queston has been posted to Taubes many times over though, and he himself can explain it a lot better than I can. If you're interested in the topic, this is definitely worth a read: http://garytaubes.com/2012/11/what-would-happen-if-thoughts-...