unexplained autovacuum to prevent wraparound

It appears to me that in my 8.4.0 system, autovacuum is running to prevent
wraparound contrary to the documentation. I have it set to a tables'
relfrozenxid has to get to 1.5 billion before that kicks in:

> show autovacuum_freeze_max_age;
1500000000

> show vacuum_freeze_table_age;
1300000000

> show vacuum_freeze_min_age;
1000000000

Table foo has an age just over 1 billion, still well under the
freeze_table_age:

> select age(relfrozenxid) from pg_class where relname='foo';
age
------------
1055823634

yet, I see this in pg_stat_activity:

autovacuum: VACUUM public.foo (to prevent wraparound)

One possibly interesting thing is that this seems to have started just after
I set foo's autovacuum_analyze_scale_factor to 0.01, since I wanted more
frequent analyze runs. I wonder if that could be related.

Any ideas? These wraparound autovacuums are taking up my vacuum workers so
no analyze workers can run.

Thanks!
Gordon

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