It is likely because the bank doesn’t know what checkbooks you own and use, and has no way to know it. You could have ordered more third-party checks (legal, not fabricated), and if you do, commonly you would use a different sequence starting point for those, so that you can distinguish which checkbook you used. More commonly the case for businesses but you can buy them from Intuit for example: https://intuitmarket.intuit.com/checks