I need to correct my post above. Westerdam was refurbished recently, most notably some changes to aft balconies, but I had completely forgotten that some inside cabins were also added in the process -- among these was 7051. So, the deck plans on some sites are simply outdated.
When in doubt, go with the cruise line's deck plans.
I am thinking HAL reconfigured the deck to put in some more cabins and renumbered them.