It is a points addition system, not a deduction system. It counts the points accumulated over 15 days. If you claim an airdrop today, when the scores are updated tomorrow, it is equivalent to adding your balance points + transaction points - airdrop points. For example, 2 + 14 - 15 = 1, which means you only have 1 point for that day. After the next 15 days, the 1 point from that day will disappear, and the points you add that day will be 2 + 14 - 1 = 15 points.