It’s all about mature card(interval > 21 days). The ratio of mature cards / total cards must be high enough to attain your “memory stability” goal.
High retention rate keep your mature card at mature state (not send them back to learning state), and promote your learning card to mature state. That is. high retention rate create virtuous circle. While low retention rate demote your mature card to learning card, and lead to “ease hell” as Anki user like to say.