SQL Server differential backup file size smaller than previous one ??

Question

Hey,

I have a production system running on an SQL Server 2008 DBMS, in Full recovery mode.

I take a full backup each day at midnight, a log backup every two hours, and a differential backup every 6 hours (06:00, 12:00, 18:00, but not midnight as the full is taken then).

In recent days however I've noticed that the diff backup filesize of 18:00 is smaller than the one of 12:00. And sometimes, the one of 12:00 is smaller than the 06:00 ...

I did not experience this behavior until recent days.

Reading from the Microsoft doc, the filesize of a diff backup should always be larger than the previous one, until a new full backup is taken.

Could anyone have a possible explanation what could be causing this ?

Thanks.