Cats are Obligate Carnivores
That means they must eat meat
In nature cats have existed on primarily a bird, rodent and small mammal diet. They had to work or exercise hard to capture their meal. And this meal was only about 7% carbohydrate. This also meant that they ate pretty much the whole creature.. which was 80 plus percent water an including the stomach and a good bit of the intestinal tract, which contained the predigested meals that the small creature had eaten. So, the cat is designed to eat the meat, bones, and small amounts of predigested grains and grasses. This diet is very high in protein and low in carbohydrate relative to most "kibble" foods on the market....that sums up the problem with the Standard American Diet of our cats.
Canned foods are more like it, and after you correct the equation for water, the canned foods are generally higher in protein content than kibble diets.
They don't do well on a high carbohydrate diet
Cats are deficient in an important enzyme required for carbohydrate metabolism. Humans and dogs have much more of this enzyme. Sedentary cats and some cats especially, have trouble with this. When fed a diet high in carbohydrate, they store up the extra calories in their body's reserves. But then when they need the energy, they take protein, not the stored carbs from the body's reserves.
Over time, they become obese, muscle depleted, and just don't feel well. Often they then develop flaky skin over their back and sides toward the tail and may then develop a stinky rash in the perineal area under the tail. Correcting the diet to one that is high in protein, relatively high in fat, and quite low in carbohydrates can help these kitties feel and do a lot better. The rash, once fully developed, may require specific treatment such as a "sani-shave" and topical treatments to resolve.
Kibble is higher in carbohydrate in general.
In order to take fat and protein and form it into a dry kibble, a certain amount of carbohydrate is required to cement it together.
The kibble that is lower in carbohydrate should actually feel greasy, because the carbohydrate content of the kibble is what gives you the dry kibble. When looking at your cat's diet, especially if she has certain propensities, we assess the ratio of the calories represented as carbohydrate as well as the calorie content of the foods. The following links give the calorie content of many popular WET and DRY cat foods.