问题描述
题主所提到的 “SuperMemo 官网”指的是 supermemo.com 吧？根据 SuperMemo 官方 Wiki：
On-line SuperMemo - SuperMemopedia可知：
This is one of the earliest and still surviving e-learning platforms in the world. First conceived as a 3GEMs project in the 1990s, backed with venture capital, it evolved into a pioneering http://supermemo.net project. Today, http://supermemo.net is being phased out. Supermemo.com, based on Algorithm SM-17, has been rewritten from grounds up and is constantly improving its user satisfaction scores. We hope to make it into the best e-learning platform in existence.
也就是说 supermemo.com 是基于 SM-17 的。至于为什么不是最新的 SM-18，根据：
Algorithm SM-18 - supermemo.guruItem difficulty in Algorithm SM-18可知：
The differences between Algorithm SM-17and Algorithm SM-18 are not significant except forthe way the difficulty is computed.
Algorithm SM-18 abandons the idea of absolute difficulty and brings back the incremental changes to difficulty in the direction determined by the expected performance. This may be very pleasing to our intuitions about item difficulty, however, as of Dec 2019, we have too little data to prove that the new approach is superior (or worse). True picture will emerge over time. We only see that users who tested the new algorithm tend to enjoy the granular nature of difficulty. If all we achieve with the change is happier students, the change will be worth the invested time.
总结一下，就是 SM-17 和 SM-18 之间的差异主要是难度的计算。而 SM-18 的难度计算方式是否更好，目前还没有足够数据验证。
猜测一下，大概就是因为官方认为 SM-18 相对 SM-17 的优势还未得到验证，所以不会用到所有产品上吧。