Can't debate her logic too much. Disembarkation day barely counts.
I think most listing advertise a certain number of nights rather than days. This could be the reason. Some itineraries list embarkation day as Day 0.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015