Hanyu pinyin is a pretty easy system of romanization to learn. There are very few “rules” that stray from its connection with Chinese phonemes. One of the rules is that the final iou is contracted to iu (unless there is no initial, in which case the i changes to y). Other than for the sake of brevity, I’m not sure why this rule was adopted, but sometimes we can see the original pop through as a mistake.
My younger son was writing his journal today about Alice in Wonderland, which we saw yesterday, and after him telling me that he thinks it’s not good to write pinyin when he doesn’t know a character, and me telling him that that’s one of the best uses of pinyin, he wrote how Alice, after drinking water from a little bottle, “jiòu biàn 小了”.