In general, a niddah should not postpone immersion.
According to current universal custom, after observing a disqualifying stain, a niddah must observe 7 clean days before immersion, whereas a tehorah must wait at least 5 days before observing 7 clean days.
Consider these two scenarios:
- Rochel finishes counting 7 clean days, and diligently immerses that night. The next morning she finds a disqualifying stain, and must wait at least 5 + 7 = 12 days in order to immerse again.
- Leah finishes counting 7 clean days, and laxly postpones her immersion. The next morning she finds a disqualifying stain, and must wait only 7 days in order to immerse again.
Why is the diligent being punished?