Unexpected backups

I'm using Acronis True Image Home 2009. I have selected c:\temp to backup and I have setup a schedule as "Periodically", "00:20" and "Incremental". It did the original backup. Then I made a change to c:\temp\this.txt and after 20 min it created another backup as expected. But it continued to make backups every 20 min even though I made no change and did not shut down.

I don't want to make new backups if nothing has changed.

Have I specified something wrong?

Eddy