It isn't as magical (or accurate) as it looks. It's just an extension of how various health tracking apps track food intake. There's usually just one standard entry in the database for mashed potatoes based on whatever their data source thinks a reasonable default value should be. It doesn't know if what you're eating is mostly butter and cheese.

How useful a vague and not particularly accurate nutrition profile really can be is an open question, but it seems to be a popular feature for smartwatches.