That just comes as a % related on how much (average) time people played the same game with and without the nose. 13.5 percent refers to those ~90 seconds added playtime. This means, on average the Tuscany demo has been played for 697 sec (~11 and half mins) without nose, and 792 sec (~13 mins) with.
It would be interesting to know the dynamics of the experiment (the testers were aiming to hold on as long as possible? Or they were simply told "play as much as you want"?), since the Tuscany demo is quite short, and most of people wouldn't play it for more than 10 minutes, besides sickness
