Yeah, I remember that throwing me off at the beginning. On the plus side 'under' is नीचे, and that's easy to remember because Nietzsche was the kind of guy to hide under a table.
Good tip. And to remember अंदर I think of the Latin word inter (inter murōs = between the walls).
I'd say because if it would be "that house", the Hindi sentence would use उस घर, not वह घर (as "that house" would be affected by the postposition के अंदर, वह would have to take the oblique case and become उस).