"Inability" means you can't do something because you don't have the skills or knowledge to do something, "disability" means you are not able to do something because of a physical or mental problem. For example, I have an inability to speak Spanish, because I never learned the language (but it's possible I could). A person in a wheelchair who can't walk, however, needs an elevator because s/he has a disability that prevents walking up stairs. When talking about strength, we use "inability" because some things are not possible for the average person to do (e.g., I have the ability to lift a pencil, but an inability to lift a car). This only changes to "disability" if someone can't do something that a majority of other people can do, or that the human body was designed to do (such as walking).