Since my English isn't very good, I wonder if I got your question or not. First of all, 島 is noun, it isn't an adjective, and it's read SHIMA or TOU. Besides, in this case, they are a part of the names of islands, it's been called in that way for a long long time, so I don't know why some are read SHIMA and the others TOU. The same happens for 町, some are called MACHI and others CHOU. However, as one word, 島 is SHIMA, 町 is MACHI. (Ex. わたしはしまにしゅんでいます。わたしはまちにすんでいます。) I hope you get it even though a bit, and I really hope other people explain about this question better than me.

Japanese vocabulary has several kind of pronunciations for a certain word as English has many synonyms for a significance. The various pronunciations are due to mixing of indigenous pronunciations and ancient Chinese pronunciations which are also very era to era. Concerning 島, しま is indigenous and とう comes from Chinese pronunciation "dao." // Could someone rewrite my comment above into decent English?