How does this work then?

These are the Casio Park Locks with the short pound between them.  Above the top lock is a sluice to divert excess water around the  two locks, returning it to the canal below lock 76.  On the lower lock - visible in the distance on this photo - we are instructed to empty the lock on leaving.. The top lock, however, is operated in the orthodox manner. If these instructions are followed then every time a boat passes up the locks they empty two lock of water from the pound but only empty one lockfull in. There is no other supply of water to this pound as far as I can see.  So why doesn't this pound end up empty?