English, for example: Bird, fern, turn & learn all have the same vowel. But are written very differently. Because the language continued to develop after the writing system was formalized.
And by develop, try and think of how different Australian English or Southern Drawl sound compared to their parent tongue. There are apparently theories of how this vowel precession works, but the point is while we can invent, bend and twist our languages with slang and colloquilisms, we're more or less stuck with the alphabet.
Possible exceptions include languages like Serbo-Croat and Azeri, where you can write the same language in different alphabets, like Arabic, Latin and Cyrillic. But that's not exactly what I'm talking about. I'm more talking about how you can't fundamentally change your alphabet to suit your needs. jabba jabba, foo'.