The number of thumbnail images may not match up with the number of downloadable images if you are disconnecting from a live call and reconnecting to the same call (with the same 9-digit Blitzz Session Code).
The tally kept in the thumbnail gallery is a running count of how many images you have taken during the current live call, not necessarily all calls associated with the session ID. The full number should be visible in the "Action" list when you open the call details.
Another thing to consider is that the number displayed will also include the location (Google Map snapshot if enabled).