Mzungu came, 'civilized' us and left us with new order for everything-including names. Before he came, I understand your name was the name they referred you with at a particular time!
They left us confused, maybe because we didn't quite customise the surname to fit our culture.
Take the example of John Njeru Kimathi. He is John Njeru son of Kimathi. He refers himself as Mr Kimathi.
His wife Kwamboka refers herself as Mrs Kwamboka Njeru. Their daughter Beryl is Beryl Kawira Njeru.
Hence in one family we have Mr. Kimathi, his wife Mrs Njeru and their daughter Miss Njeru. Collectively, the Njerus(not the Njeru's)
See the African confusion?