Dear Federica,
Unique Visitor is based on Browser User Agent. Whenever new Browser User Agent is identified, visitor ID will be placed as a persistence cookie to map the Unique Visitor. Till you clear your cache, same Unique Visitor will be mapped back.
Bounce Rate is based on Single Page Visit. Single Page Visit is formulated when entry page is also the exit page i.e. Single Server call in a visit.
Thank You
Arun