Hello Will17,
In your case, 'on' is used to refer to a flat surface.
A tabletop is a horizontal board to put things on.
A coffee table is a horizontal board to put things on.
A shelf is a horizontal board to put things on.
A jewelry case is a box to put things IN.
Personally, I would not say you are wrong but whether it is necessary depends on how specific and descriptive you want your definition to be.
A shelf is a board.
A shelf is a horizontal board.
A shelf is a horizontal board you can put things on.
A shelf is a horizontal board you can put books, toys and food on.
Hope that's clear
Calvin