use of pg_stat_database

Hellow,
 
I like to track the number of committed transaction for my database
So I use the following command
Select * from pg_stat_database;
 
The column xact_commit gives me the number of transaction committed
 
But if I execute the same command another time, the column xact_commit gives me an other number.
 
Between the first select and the second select I don’t execute any transaction on my database
 
Please, can any one help me to understanding the cause of the change?
 
Thanks very much