When an authoritative DNS record changes, recursive resolvers may continue returning the previous answer until its cached TTL expires. Resolvers that did not cache the old value can see the new answer immediately, while active caches legitimately disagree.
What TTL controls
TTL is the maximum time an answer may remain cached. Lowering it before a planned change can shorten the transition, but only after resolvers have received the lower TTL. Changing a five-minute TTL at the same moment as the address does not invalidate an older one-day cache.
What a global checker measures
A propagation checker asks multiple probes or recursive resolvers for the same record. It is a sample, not a census of a country or the internet. Compare the exact answer, status and TTL at each location, then repeat only when the expected cache window has passed.