Let's move away from the frozen dinners for a moment because apparently you're getting really confused about water somehow magically being added to a dish just by freezing it. I don't know where you think this water comes from if it wasn't there already.
This method works for just about any other packaged food. The serving size is given in grams. For example it could say 1/4 cup is 28 grams. So if you meausure by eye 1/4 cup, then you're going to think you're getting the amount of calories that are present in 28 grams, but in reality you could have just measured out 31 grams.
28 grams of almonds is 162 calories. 31 grams of almonds is 179 calories. Maybe not a huge difference but if someone keeps making mistakes like that all day, they will add up fast, and then they could be confused as to why they're not losing any weight.
My point is that the gram measure is always more accurate than the volume measure. If you're plating up your food and measuring everything by volume, whether it's cereal, nuts, vegetables, etc. and not weighing it by grams, then your calorie count on paper is going to be different from what you actually ate. 1 cup or one package of something is always going to be a less accurate measure than if you had weighed the food.