When viewing the Camera Roll in a friends iphone I am able to see many pictures. However when viewing the same folder in Windows Explorer I can only see a small number by comparison. My friend uses iCloud also and syncs to it and the pictures appear to be all there. The issue is that she wishes to copy the pictures from the phone to local PC storage to clear space on the iPhone memory.
If on the iPhone you choose to optimize storage of your iCloud Photo Library, then only some of your photos are stored in full resolution on your iPhone. That explains why you can only see a subset of the files from the Windows Explorer.
The rest of the photos are stored in high-resolution in your iCloud account only. You can access them at www.icloud.com from your Windows computer.