I wondered the same thing. Here's what Wikipedia says: "The Tropic of Cancer position is not fixed, but varies in a complicated manner over time. It drifts south almost half a second (0.47″) of latitude per year (it was at exactly 23° 27′ in year 1917)."
Right. And in that year, the nation of Chad was not touching the Tropic of Cancer. That will happen again at some point, and because of that fluctuation, Chad is not included.