question regarding gather schema statistics

I have setup the cron job to run gather schema statistics once a month.

exec dbms_stats.gather_schema_stats(ownname =>'QC_SYS',cascade => TRUE, method_opt => 'FOR ALL COLUMNS SIZE AUTO', estimate_percent => dbms_stats.auto_sample
_size, options => 'GATHER AUTO');

However, when I checked select table_name, last_analyzed from dba_tables where owner='QC_SYS';

Some of the table showed last analyzed as this month, some showed last month. some showed as April of last year. My question is how does the gather schema stats pick which table to analyze? I thought it would analyze all the tables under the schema. Gurus, could you please explain? Thanks.