Africa's Children has a distinct, special strand of DNA that 9 markers is sufficient to distinguish an African ethnic group(s), if my understanding is accurate. But, I'll leave the in-depth explaining to Toot, the expert. She did a great job explaining this in a previous post with a reference to an article.

Also, you have to consider that some groups intermixed. The Mende and Temne intermixed with each other; therefore, it is not surprising that some people's DNA sequence will match persons in both groups. Same as the Fulani and Hausi of Nigeria...mixing going on. Some people will match both of those groups as well, and this applies to a number of other ethnic groups. You may be fortunate to have a DNA sequence that is an identical match to people of one ethnic group. Sometimes, you may have two matches, and if they are Mende and Temne, using them as an example, you basically are now aware that some of your ancestry hailed from Sierra Leone.

Just my 2 lil ole cents worth,
Mel