db stats vs table stats

I've been periodically collecting the stats stored in
pg_statio_all_tables and pg_stat_database for ~30 different
clusters, and have noticed a curiosity.

I would have thought that for a given period, the change in
pg_stat_database.blks_read would be <= the sum of the changes in
pg_statio_user_tables.heap_blks_read +
pg_statio_user_tables.idx_blks_read +
pg_statio_user_tables.toast_blks_read +
pg_statio_user_tables.tidx_blks_read.

In short, the total would be <= heap + idx + toast + idx for user
tables.

It does not appear that way.  The table-level IO stats appear to
be typically 1-2 orders of magnitude larger than the db-level
stats.  Can anyone explain that?

TIA.
Ed