The top is an actual part of the object: A box has a top, a bottom, and sides. At the top marks a point location, like at the station-- it is a relatively small point viewed from relatively far away (in the speaker's mind). On the top is a 2-dimensional location, like on the table-- it is a surface, viewed at a nearer distance than 'at'.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.