Regardless of how many selections you have on the timeline from the same video/audio media event, they all hark back to the original event in the media file folder where the media event is held. That you have trimmed an event down to, say, 10 seconds from a 5 minutes event does not create a separate event as far as Project Media is concerned. If you've decided to delete a trimmed event from the timeline but want to leave other trimmed timeline selections from the same clip, then the original event must be maintained and not deleted otherwise they all timeline selections go. If any event on the timeline relates back to a media event, then that media event must be retained. A way to get around this is to first render out your trimmed events so that all your selections are new individual media events and the unwanted media events can be deleted as desired. But hopefully the original will be stored elsewhere just in case.