I think we are “chasing our own tails” here. 
I have read that erythritol can be from 0 to 0.2 calories per gram. I don’t know, but I wonder if there are different formulations of it. Or, different rules for measuring and/or reporting, in different places?
There are at least two different cases here. One is what I think is by far the best explanation - that the company used information from it’s other tonic waters, which actually do have 11 grams of carbohydrates in them. The “Light Tonic” would not actually have that much; they just have the wrong information on the label. 16 calories from the added sugar, and 4 more from other stuff - we do in fact end up with 20 calories total.
The second case is that somehow there are 7 more grams of carbohydrates in there. Erythritol doesn’t necessarily get counted as all carbs. Some people don’t count it at all. Some count all of it. Some count half of it, as with:
https://dtc.ucsf.edu/living-with-diabetes/diet-and-nutrition/understanding-carbohydrates/counting-carbohydrates/learning-to-read-labels/counting-sugar-alcohols/#:~:text=Sugar%20alcohols%20are%20still%20a,sugar%20alcohols%20under%20total%20carbohydrate.
“Because sugar alcohols are hard for the body to digest, the effect on blood sugar levels is less than standard sugar. When counting carbohydrates for products made with sugar alcohols, subtract half of the grams of sugar alcohol listed on the food label from the total grams of carbohydrate.”
I had never heard this - just saw it right now when searching for information. But I really don’t think it matters - I’m pretty sure they just messed up the label.