I don't know why Kathy but sometimes the measure is really wrong. For example I got those packages of purple potatoes in the produce section. It said that one serving was a certain amount of potatoes and then it had a gram measure. The number of potatoes it took to get to that gram measurement was always less than the number of potatoes stated. Also people can be inaccurate when they measure something by eye in a cup. I figure it's pretty common for people to play games where they think "well if I add a little more it won't go over this line", etc. whereas it's harder to cheat yourself or mismeasure if you weigh it.
Most of the time the gram measurement as weighed by the scale versus the "one whole package" serving size is pretty similar, but it can be off by enough to cause a difference in calories that's significant.
This thread is mostly for people who really are trying to count their calories but just aren't seeing any progress. Sometimes all you need is a more accurate method of counting.
I also think I figured out what that water is that you're talking about. When water freezes, it expands, and if at some point it melts again, it's going to leave a pool of liquid on top of the food. It wasn't added to or taken away from the food (there's no additional liquid created by the freezing process) but came from the food itself and is part of what is intended in the gram measurement even though it doesn't have any calories in its own right.