Inconsistent rows returned & examined in slow query log

[James Harvard]

I have entries in my slow query log for identical queries but, as you can see from the log
entries below (including one irrelevant query), the number rows examined and returned
varies. The tables are _not_ being updated.

The query cache is 'on demand', so I'm also not sure why the subsequent queries were not
dealt with by the query cache.

Any ideas?

Thanks, James Harvard

# Time: 060103  9:45:12
# User@Host: lasso[lasso] @ localhost [127.0.0.1]
# Query_time: 86  Lock_time: 0  Rows_sent: 12  Rows_examined: 6733255
select sql_cache dates.date_month from data_gb_e data inner join dates on data.date_id =
dates.date_id and dates.date_year = 1998 group by dates.date_month order by
dates.date_month;
# Time: 060103  9:45:46
# User@Host: lasso[lasso] @ localhost [127.0.0.1]
# Query_time: 70  Lock_time: 0  Rows_sent: 7  Rows_examined: 3737912
select sql_cache dates.date_month from data_gb_e data inner join dates on data.date_id =
dates.date_id and dates.date_year = 1998 group by dates.date_month order by
dates.date_month;
# User@Host: lasso[lasso] @ localhost [127.0.0.1]
# Query_time: 50  Lock_time: 0  Rows_sent: 4  Rows_examined: 1585713
select sql_cache dates.date_month from data_gb_e data inner join dates on data.date_id =
dates.date_id and dates.date_year = 2000 group by dates.date_month order by
dates.date_month;
# User@Host: lasso[lasso] @ localhost [127.0.0.1]
# Query_time: 117  Lock_time: 0  Rows_sent: 9  Rows_examined: 5196480
select sql_cache dates.date_month from data_gb_e data inner join dates on data.date_id =
dates.date_id and dates.date_year = 1998 group by dates.date_month order by
dates.date_month;
# User@Host: lasso[lasso] @ localhost [127.0.0.1]
# Query_time: 113  Lock_time: 0  Rows_sent: 9  Rows_examined: 5196063
select sql_cache dates.date_month from data_gb_e data inner join dates on data.date_id =
dates.date_id and dates.date_year = 1998 group by dates.date_month order by
dates.date_month;