If your Monthly Quota is significantly less than your number of households that you identify on a monthly basis, then during the month you may see a difference between the Daily Quota and the Daily Opportunities, creating new Daily Missed Opportunities.
If you add more mail quota budget mid-month, it will get automatically spread into the remaining days in the month and tie into the AI's decision. The AI will look back to earlier opportunities in the month and mail the most likely to buy based on the analysis.