CGNAT

CGNAT is what mobile carriers and some ISPs do when IPv4 addresses run short: many subscribers share one public address through a second layer of NAT.

For IP checks this means one address can represent thousands of people. Abuse from one subscriber can taint the whole address, and a clean history does not prove the next user is fine.

Mobile addresses are often CGNAT, which is why mobile proxies behave inconsistently: the exit address can change between sessions without warning.