I think executive dysfunction has a role in the difference too. Sometime people want to be able to do something but cant organise their thoughts or motivation enough to do that, whether they are depressed or not.
I guess lazy has a range of connotations. Some people might think it is lazy to leave a plate on the lounge floor after eating. But in our house that is usually - it took so long to motivate yourself to eat what was on the plate, you now have nothing left to put it away. I think if I'm honest, most people have a reason for their "laziness" but it is often used as a descriptor of behaviour other people don't understand. I've been looking at mood scales a lot recently so sorry if this doesn't make a great deal of sense. If your mood is usually between 6 and 4 then the amount of effort it takes to do things is not usually demanding. You might not feel like a spring clean on a 4 day, but most things aren't so much effort. So it is difficult to understand why someone who has moods higher or lower on that scale might not be able to do the things you see as normal. If it takes all the effort you have to do 3 small task, your house and hygiene are not going to be squeaky clean. Similarly if you can only focus on one project remembering to eat is a bonus. It isn't lazy to not put the plate away...that detracts from the "ever so important project" you're working on.
It might be a bit off the point, but I don't know if there is really such thing as lazy. It is an evaluation of someone else's priorities and actions. I'm not sure we ever have a full enough picture of the thought process to evaluate something fairly.