I mean if it's just single letter to phoneme then I can understand, most vowels are pronounced differently depending on if they are stressed or not, there is qu

I mean if it's just single letter to phoneme then I can understand, most vowels are pronounced differently depending on if they are stressed or not, there is quite a long lists of vowels that use two letters and then there are some consonants that change sound in the presence of other consonants. And then there's the ij, which despite what other languages think is really just one letter.