The current version of the site uses Wanikani’s APIv1, which is missing level-up times, so wkstats has to guesstimate. Some people’s data is too complex to be able to guess well, and you get strange results.
You can use the new (but not yet complete) version of the site at https://www.wkstats.com:10001/. It uses Wanikani’s newer APIv2, which has more info and gives better results.