When you learn a language there are always quirks and inexplicables. For us who have grown-up speaking English we know to get in a car and on a bus rather than getting on a car and in a bus. Some one once explained this was most likely an artifact of the original open top cars. No longer a reality it is now a frustrating quirk for those who learn to speak the English language. Then of course there is live and live or there and their or two, too and to all of which I see little logic in.
With the Mandinka language there is the word Kontong which can mean the action of greeting people, eating lunch or the noun referring to your last name. Beyond being very confusing, once you understand their culture perhaps there is something deeper going on.
On my own logic your last name is your identity (more so than just your name). You are identified as having certain characteristics (similar to how we associate an Irish name or perhaps Dutch or german).
From here it is easy to arrive at the meaning for greeting particularly as it is most often used as a "say hi to the people in your place".
Now as for lunch, here you begin to understand the importance of eating together. In Gambian culture it is almost as if you have not visited someone until you have eaten from their bowl.
As I have returned and begin to visit my many friends from over the years I have become accustom to the need to eat meals in all those places - this cultural act shows I feel confortable and at home. Naturally it also ensures I am also well fed.
Speaking of which a plate of nyan ka tan has just arrived. I should get to visiting. Here is a photo.