حَتَّىٰ يَطْهُرْنَ / حَتَّىٰ يَطَّهَّرْنَ
Until they become pure / until they purify themselves.
In the discussion of menstruation, one reading emphasizes the cessation of bleeding, while the other points to purification through washing. Instead of rejecting one reading, the juristic treatment allocates them to distinct circumstances.
The verbal form matters: becoming free of bleeding is not the same description as performing the act of purification. Applying the readings also requires the rules governing the duration of menstruation.