You asked this just to smoke me out, didn't you?

The simplest answer would be differences in language, but that's not quite all. In terms of language, the prefix for the kings of Kongo was mani-, and in variants of Sotho and Tswana the root is kgosi while in Luvenda it's khosi. But that may also get at the rest of the point, which is that the two words are not the same grammatically either. mwene-/mani-/munhu-/etc create constructions like manikongo or mwenemutapa, the holder of those places, whereas kgosi (for example) creates constructions like kgosi ea baRolong or chief of the Rolong people. In Luvenda, heavily derived from vaShona (chiShona), seTswana, and northern seSotho, it's khosi ya vhaVenda. My user handle is an archaic form of that used for "great chiefs," which is more correctly khosi khulu ya ___ but honestly was not used as a formal title until the 20th century. (Luvenda also has a title for king now--thovele--which however was reserved for the divine in the past.) In Zulu, it's inkosi enkulu, sometimes with an h in there historically, sometimes not.

Usually such titles in Nguni and Sotho-Tswana languages (as well as in Luvenda) indicate power over/among people rather than a specifically created title over a state entity or locality. [edit: this has begun to change in more recent periods, as people became co-identified with particular spaces.] The nouns denoting kingship or chiefship in vaShona, for example, are mambo and she respectively. The noun kosi, in the language, refers to relocation or movement of people, which may be a link; I have not however seen historical evidence of a drift in meaning (like "mover of these people") as languages spread to the south, as tantalizing as the thought might be. Literally translated, mwene on the other hand refers to anyone who has the right to a place or thing, and it is closer to "proprietor" than "ruler" per the vaShona project. Kikongo also has different nouns referring to leaders than the particle mani, and records a similar meaning to mwene for it.

It may be worth pointing out that in the South African languages--all of them, really--the titles given to kings were simple modifications of the same titles given to chiefs of smaller and less prominent houses, and sometimes even those weren't really used until the modern era. In some, you have other titles for household heads in different lineages (in Luvenda, those are the magota, village chiefs) but all of the great chiefs and even kings were mahosi, dikgosi, amakosi, or whatever. So it's a function of origin/grammar as well as meaning, although the specifics of that origin are really hazy.

If anyone is a native speaker or a linguist of S-group languages and knows more, please do share--I only have a partial grasp of two of these languages beyond the grammatical structure. So there may be insights embedded into the broader tongues that I simply do not have.