My daughter does this, has done since she was a little girl. Once, I asked her why - she just shrugged and said she preferred it that way, no specific reason.
Fair enough, I thought. Her books, her shelves, no one was getting harmed by it - and I moved on.
Why is everyone getting so butt hurt in this thread about it?
The latter. The buildings on the right are constructed under a shelf of rock - on the left, they've built up to meet that shelf.