It should be noted that almost every African (on both the leaving and receiving end) does this. So, if one does actually arrive on "American time," don't be surprised if a party is still getting set up, a person is still getting ready, etc etc.
Wedding invite says, "Note: There will be no African Time at this ceremony. It will start promptly."
You arrive at a party on time (craziness!) and half the food isn't done, the hostess is still in her cooking clothes, etc.