Pronunciation of 对 and 水
After [...] a consonant, iou, uei, and uen are simplified as iu, ui, and un, which do not represent the actual pronunciation.


https://en.wikipedia.org/wiki/Pinyin#Letters

This spelling seems to have been inherited from the earlier Latinxua Sin Wenz, itself following the use in Wade-Giles. Such simplifications were likely used since [wi] is an allophone of [wei]:

In SC [wi] is a variant of [wəi]... SC does not make use of such contrasts as [wi] vs. [wəi], or [ji] vs. [i], which English does (consider we vs. way, and yeast vs. east).


A Phonology of Standard Chinese


Related questions:


Then we also have sounds that are there but simply left out, such as -ui actually being pronounced -uei (水, 对, 贵) and -iu actually being pronounced -iou (六，休，牛).


Is pinyin an accurate pronunciation guide?
Why is it written LIU in Pinyin, when there&#39;s clearly an O sound?
Are there rules about the differences between pinyin spelling and pronunciation?