When water freezes in a freezer, a lot of air is frozen along with it. But there are also glaciers, huge ice sheets hundreds of feet thick. They are so heavy that a lot of that extra air has been pressed out. That makes the glacial ice a little bit heavier than the ice from your freezer. The biggest difference from one piece of ice to another is it's structure.
Ice is a crystal.
A crystal is just a fancy name for when the atoms and molecules are formed into an organized shape. A good example is a brick building. The bricks are laid so that they are in a pattern. Every crystal has many defects in it. When you let the ice take a long time to freeze, then it is shaped better.