Why some of my photos are not shown in iCloud after the successful back up

You cannot view the contents of an iCloud Backup. You can only use the backup to do a full restore of a device or to setup a new device.

The photos that can be viewed at www.icloud.com are photos that were synced via iCloud Photos. See: Set up and use iCloud Photos - Apple Support

Syncing is dynamic (updated automatically for new photos, deleted photos or edits to photos) and not triggered by an iCloud Backup. You can view the status of syncing by opening the Photos app on your iPhone, tap Library and then scroll to the bottom of the screen.