Asking a questionable customer a follow-up question (like their zip code or birth year) can help because:

Asking a customer with a questionable ID a casual follow-up question — such as their zip code, birth year, or address — can help because a person using a fake or borrowed ID may hesitate, become flustered, or give an answer that does not match the information on the card. This is a supplementary technique, not a replacement for examining the ID itself. If the answers raise further doubt, or the ID still cannot be verified as valid and matching, the server should decline service. Such techniques support careful age verification.