'The top' is a location (and of course, part of something else).

(1) If 'the top' has adequate dimension, then a person can be 'in the top 10% of her class', 'my corkscrew is in the top of the wine bottle'. It is within or inside the range or depth of 'the top'.

(2) If 'the top' has an upper surface which is of interest to or focus for the speaker, then she can say 'the wine bottle is on the top of the TV', 'they mounted the aerial on the top of the building.

(3) If the dimensions or planes of 'the top' are not important to the speaker (usually it is very big or far away-- so it is just a point location-- or perhaps conceptual), then she would say 'I left my backpack at the top of the mountain', 'I am at the top of my English class'.