>> 3. Select "Cruise Personalizer" from the menu bar.
>> 4. Enter your first name, last name, and booking ID in the spaces provided, then select the "Go" button to the right of the space for the booking ID.
This will retrieve your booking information from Princess'a computers. You also can enter all your precruise information (immigration, emergency contact, etc.) and reserve shore excursions in your ports of call through the cruise personalizer.
I have no first-hand knowledge, but I can speculate as well as you can.
>> The most likely scenario probably is that some travel agent had a large group booked on another cruise line that had to cancel a sailing for the same week, so the travel agent moved the whole group onto MV Grand Princess for the dates of your cruise when she discovered that the ship had enough space available.
>> Another possibility is that a couple decided to marry aboard the ship on relatively short fuse, put out the word, and filled all remaining space with wedding guests.
>> It's also possible that some travel agent "dropped the ball" on a group booking, had to "scramble" to accommodate the party, and ended up putting the whole group on your cruise because that's where space was available.
>> Seeing that the ship had a lot of open space, Princess might have offered your cruise to travel agents as a "familiarization" cruise (either very steeply discounted or free) and ended up with enough takers to fill the ship.
I doubt that these are the only possibilities, but they are the possibilities that come to mind immediately.