Why are words like 天、面、先、盐 romanized in pinyin as tian, mian, xian, and yan, even though the ending is pronounced [iɛn], and would be better represented as -ien? Likewise, why are words like 卷、全、选、元 romanized as juan, quan, xuan, and yuan, when the final is pronounced [yɛn] and would be better represented as -uen? (at least the way northern Chinese people pronounce it – it seems that southern Chinese people pronounce it as [yan])
Is it that those words actually were pronounced with a [ian] and [yan] final in the past? That doesn't seem like a complete explanation, as Wade-Giles uses -ien, and Wade-Giles is from 1892 (although it does use -üan and not -üen).