Not all image files on your storage will be photos taken by your Camera app. Certain apps may cache images in their own app directories (for quick use later), and if the app developer forgot to create a .nomedia file in that directory, then those images will show up in your gallery app. This might account for some of the images that you don't recognize.
The Google Photos app (and some stock Gallery apps) goes a step further and will display photos that have been backed up to your Google Photos in the cloud. You can tell which photos are in the cloud by going to photos.google.com on your desktop browser and seeing what's there. These photos are accessible from your phone when it's connected to the web, but aren't downloaded to the phone -- so just because you see them in the Photos app, it doesn't mean they're taking up valuable space on the phone. This can be very important for people who take tons of photos, because they can easily back them up to the Photos cloud, and then delete the device copies so that they don't run out of space on the phone. Remember that backing up photos to Google Photos isn't actually "syncing," as you're describing. You can still delete the device copy and keep the copy in the cloud by selecting the photo, tapping the Menu button, then Delete Device Copy. (If you tap the garbage can icon instead, that will actually move the photo to the Trash, which includes both the cloud copy and the device copy.)
Clearing the Google Photos cache won't get rid of the photos in your Google Photos cloud. The cache is only temporary, and gets built up again by the app as you use it. If you only want to see photos that have been saved to your phone, then open Google Photos, tap Menu>Device Folders, and select Camera.
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