"I am sitting at the table."
Translation:Mi sidas ĉe la tablo.
"sur la tablo": You're sitting on top of the table. "apud la tablo": You're sitting, and the table is so close you can touch it, but you you may not be turned towards the table. "ĉe la tablo": You are sitting close enough to the table to use it, with your body turned towards the table.
Because it is not about direction, but about the position.
THE DIRECTIONAL -N
In addition to its use for the direct object, the -n ending is also used to show direction:
Ŝi saltas sur la tablo. = She jumps (up and down) on the table.
Ŝi saltas sur la tablon. = She jumps onto the table (from another location).