Subject Reading data from MON$IO_STATS

We're developing a project which uses Firebird 2.5 embedded.
There are several connections to a database.

When we select some statistics from the database we use the query "SELECT MON$PAGE_READS, MON$PAGE_WRITES, MON$PAGE_FETCHES, MON$PAGE_MARKS FROM MON$IO_STATS WHERE MON$STAT_GROUP=0"

But it seems that every connection has different statistics for the STAT_GROUP=0. But there is in your documentation written about groups "0: The database as a whole."

Also there is written "When the database is started up the cache is empty". What does it mean when first or every connection is started up?