If one occurrence of qurūʾ were required to mean both menstrual periods and periods of purity, the method of counting the waiting period would be unclear. Each sense supplies a different legal measure.
Choosing menstruation connects the count to menstrual cycles; choosing purity connects it to periods of being ritually pure. These are not interchangeable measures.
Juristic arguments seek evidence favoring one sense. One cannot make both independently assigned senses mandatory merely by appealing to caution.
This demonstrates that lexical ambiguity has direct consequences for legal interpretation and practice.