autovacuum and transaction id wraparound

Hi,

I have a question regarding the following snippet in the official Postgres
documentation:

"If for some reason autovacuum fails to clear old XIDs from a table, the
system will begin to emit warning messages like this when the database's
oldest XIDs reach ten million transactions from the wraparound point..."

Do you know any real reason why the autovacuum may fail to clear old XIDs?

Is this highly probable ?

Thanks,
Pawel

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