Near as I (as a non-native speaker) can tell, "isle" and "island" are pretty much synonyms. However, that only applies when you use them in a sentence; when you use them in a name, it's a different matter. I mean, you wouldn't call that thing to the North of Belgium "Lowlands" (even though "low" is more or less a synonym for "nether")? Nor would you call that thing to the South of Canada "United Provinces of America".
Of course, these examples are specific to English, but the phenomenon exists in every language (well, probably).
Long story short, it's a natural language (as opposed to a formal language, such as mathematics or programming languages). Any attempt to force them to make sense is doomed. Okay granted, some natural languages are more consistent than others: from what I understood, Finnish is very consistent and indeed, English is a horrible mess even for natural language standards ("He is a quick worker" -> "He works quickly" but not "He is a hard worker" -> "He works hardly", seriously...!?), but it's being used by people, who can cope with slight irregularities (programming languages are also used by people of course, but in that case, the needs of the compiler trump the needs of the programmer).
