Oil is placed on the pan and then it is heated to form a non-stick coating. This layer can have small holes in it, so the process is repeated many times. The holes do not line up, which makes the path for water to get through much longer or blocks it. This means water can not easily make it through all of the layers. That also means any water that gets in can't easily get out, and it can cause rust to form if it makes it through the coating and is left on there.