It could be that water is part of the freezing process, but let's assume it is not. That leaves just the liquid that somehow 'leaks' from the food itself. It was part of the food, stays part of the food, albeit frozen stuff you see pooled around the frozen food when it melts. The only way that food would 'gain' calories is if calories were added, with some additive that is not listed on the list of ingredients. In a nutshell, that is my point - that nothing was added, and if it were added, it would most likely be water during the freezing process.
In your example, you take something that is reported to weigh 255 grams but when you weigh it, it weighs 270 grams. There are two things that could be happening - water somehow was added during the freezing process (no calories) or the manufacturer allows for some variation in the weight of the food. The manufacturer is extremely motivated to observe strict portion control - that's how they make their money, controlling portions when they turn out product from the raw materials they have. So... to make sure that is not water, just thaw and drain the frozen meal, if you can. then weigh it. Most likely, it will have some gravy or some liquid so that's difficult to do and still keep the integrity of the dish. Even better, when you drain the meal after thawing, cook the liquid and reduce it by half, on top of the stove. You will have a more intensely flavored sauce. You add it to your thawed meal and bake it in the oven. If you weighed the whole thing beforehand, you'd see that it is even lighter (less water, because it cooked off, but no more calories than you started with!)