Exactly what FitDay does to cooked veggies is a bit of a mystery.
Often the website assumes some added fat and salt. Sometimes changes in water content will change the % of various nutrients, but quite how that is calculated into the nutrient contents is not clear.
The best thing to do is to use the raw value and add any other ingredients you use in the cooking proccess. Raw onions sauteed in butter. Add onions, raw and butter, 2tbsp (or however much you think is absorbed).
Hope that helps!