Without explaining all the math, because math just isn't everybody's thing, here's the ideas that lead to all that math:
If you put a tablespoon of sugar into a glass, you have 100% sugar in the glass. The contents are completely sugar, 100%.
If you put a tablespoon of sugar into a glass, then add a whole bunch of water, the contents are mostly water and only a little bit of sugar, right? So you might have only 2% sugar in the glass, and the other 98% is water. So now you have a low-sugar, low-carb drink, according to the math, right?
If you pour it into a bigger glass, and add more water, you might now only have 1% sugar, so now it's even lower, right?
EXCEPT....no matter how much water you added, you still have a tablespoon of sugar in that glass, and you are still consuming a tablespoon of sugar.
Wet cat food is usually about 80% water, which makes the food content numbers, especially the carbs, look low. All the math you see on calculations takes out the water, and then evaluates the food that's left, as if it were dry and only the food part. And that's why you can't just read the numbers off the can, they include all the water in the mix, which kind of hides how much of each nutrient is actually in there, just like adding more water to your glass of sugar.