I figured it was based on the time and the rating (ex, moderate) and MAYBE personal info like weight but all in all a very loose estimate & not accurate. There's no way it really could be.
I count calories I eat but not usually exercise calories.
Although you can kind of get an idea what you're burning if you study the different lists of calories burned activities online or in myfitnesspal.
If you want greater accuracy & less effort maybe look into one of those fitness trackers that attaches to you & monitors your vitals.
Sweat doesn't neccesarely mean burn calories, sweat means your body is too warm so you need cool dónde and thats where you start sweating. Maybe you are in a cold enough enviroment so, even if you are workingout, your body isn't getting warm enough so you have to sweat, but you're burning calories.