This is a great photo. It's me with my sisters. We're in the garden at home. At first, I was baffled how on earth can one be at home (ergo inside) and in the garden (which is probably outside) at the same time. Then I came to conclusion, that perhaps, when you say it, you mean that you're in the garden which is just by your house. Do I get this right? If that's the case, why the definite article isn't enough here? Surely, if we say to someone "in the garden", doesn't it mean that we assume that our interlocutor already knows which garden we are talking about?