Dave - you totally just hit the nail on the head...
If you go to the Seabourn web site you can see they sell multiple consecutive itineraries as one cruise... including the cruise this couple was going to sail upon next ... (you can't look up cruises that have already started)
Using this page as an example - Seabourn gives passengers who want consecutive cruises the option to buy them as a single cruise of longer duration - beginning on the same sail date.
Although it has been reported she was on three "back to back " cruises, in fact Seabourn will sell that same package as a single cruise. Here is an example:
Cruises to Arabia, Africa, Europe, the Caribbean and the world on Seabourn, one of the best cruise lines