WMI Event Watcher Task Continual Firing Event When Not Triggered

I have been testing with the WMI Event Watcher Task, so that I can identify a change to a file.
The WQL is thus:

SELECT * FROM __InstanceModificationEvent within 30
WHERE targetinstance isa 'CIM_DataFile'
AND targetinstance.name = 'C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Backup\AdventureWorks.bak'

This polls every 30 secs and in the SSIS Event (ActionAtEvent in the WMI Task is set to fire the SSIS Event) I have a simple script task that runs a message box).

My understanding is that the event polls every 30 s and if there is a change on the AdventureWorks.bak file then the event is triggered and the script task will run producing the message.
However, when I run the package the message is occurring every 30s, meaning the event is continually firing even though there has been NO change to the AdventureWorks.bak file.

Am I correct in my understanding of how this should work and if so why is the event firing when it should not ?