Why did the USA use Bunyoro?

I was checking this link  http://www.state.gov/r/pa/ei/bgn/2963.htm from by the USA goverment about Uganda but i failed to underasynda why they used Buyoro instead of Banyoro.

Under the section of People (about people in Uganda), they have this info

People
Nationality: Noun and adjective--Ugandan(s).
Population (2007): 30.9 million.
Annual growth rate (2008 est.): 3.3%.
Ethnic groups: Baganda, Banyankole, Bahima, Bakiga, Banyarwanda, Bunyoro, Batoro, Langi, Acholi, Lugbara, Karamojong, Basoga, Bagisu, and others.
Religions (2007): Christian 85%, Muslim 12%, other 2%.
Languages: English (official); Luganda and numerous other local languages. Swahili is generally spoken only among the Ugandan military.
Education: Attendance (2008; primary school completion rate)--54%. Literacy (2003)--70%.
Health (2008 est.): Infant mortality rate--78/1,000. Life expectancy--51 yrs.


If you can note, they used Bunyoro instead of Banyoro, any idea?