Retrieve plugin is fired multiple times

I wrote a plug-in and when I register it with "Update" message, it's fired once. However, when I register with "Retrieve" message, it's fired multiple times. This is my set up:

Btw this is the plug-in was fired three times in a row (with the same depth = 1). If I set the Excution Mode from Sync to Async, the plug-in will be fired 6 times.

Any advice, please?