u/IAmBanEvading

It was said that Arayashiki prioritizes deleting the user's most "precious" memories first.

So the fact that Ryoshu completely forgot that she stabbed Shiomi on the heart means that was one one of her most precious memories.

Ryoshu's "top 3 best things to ever happen to me" list looks like:

1. Araya
2. Killing my mom
3. Saru