If an image is in the photo stream it is not in the local Aperture library which is why when you plug in your phone it imports the image. In this case it is importing it into the library. It says it is adding it to the photostream because you have the option in Aperture set to add new images to the photostream and for Aperture this is a new image.
These options can be changed in Aperture->Preferences->Photostream
For what it is worth I find it can get confusing also so I turned off all the automatic Photostream options and if I want to add somthing to the stream or add something from the stream to my Mac I do it manually.
Of curse you have the option of turning Photostream off entirely, you don;t have to use it for your images (or for anything else for that matter).
so as long as I have my photostream turned on, all photos taken by my iDevice (if they haven't been download from the mac via network) ,when connected to the mac, will be sync to apearture with event's name called "XX month photostream" (though actually they comes from the usb wire not the net) , right?
Here's the settings Frank refers to.
If you have Automatic Import enabled (checked), when you open Aperture, it will automatically import new photo stream photos to the project you mentioned named by month of the year. Uncheck this if you don't want this behavior.
The other option, Automatic Upload, will add any new photos you import into Aperture (via a memory card or USB connection, for example) to photo stream.
If you keep Automatic Import checked, there's probably no need to ever connect your iOS device to the Mac to import its photos.