This oddity is an example of assibilation, which describes the process when a non-hissy sound is turned into a hiss by a speaker.
The name Kiribati started off, you see, as the local pronunciation of the island group's colonial name Gilbert.
The same thing has happened all over the place, in all sorts of languages, including English and various Latin-based languages. The English and French -tion or Italian -zione ending of words, for instance, started out in Latin as something pronounced tee-oh, but which is now in English sadly reduced to 'shn.
Germanic languages have done their bit towards assibilation, too: the ss in Wasser, wasn't originally there (as can be seen by its English equivalent, water).
This process carries on today. In some southern American dialects a th sound at the end of a word is assibilated, so that bathroom becomes something like barssroom, and birthday berssday.
Slightly stretching the idea (I think) are examples which count the sound ch (as in church) as a hissing sound, or sibilant. Here we get the mechanism for the word dyke turning to ditch, as well as a strong tendency in all of us to say the word nature naycher instead of naytyer.
And of course in any discussion of assibilation no one can forget the undoubted influence of dentures...
Thing To Use Today: assibilation. This word is, pleasingly, itself an example of assibilation because the word was assibilatio in Latin. Sīlibāre means to hiss.