Finance reported 1.8 million active cardholders last month, but your SQL gives 2.1 million, and your boss needs the number for a meeting tomorrow. You have an accounts table and a transactions table; one customer can hold multiple cards, and transactions include refunds and declined authorizations. How would you investigate the gap, and which number would you report?

财务报上个月活跃卡户180万，你用SQL算出来是210万，老板明天开会要用这个数。客户表和交易表都在，同一个客户可能有多张卡，交易里还有退款和被拒绝的授权。你会怎么排查这个差异？最后报哪个数？