JPA update entities every (even empty) commit

[bardolf]

Hi all,
I have following question. Why JPA call updates on all changed entities 
during commit even though persist wasn't called.

[snip]
EntityManager em = 
javax.persistence.Persistence.createEntityManagerFactory("JavaApplication3PU").createEntityManager();
Address ad = em.find(Address.class,1);
ad.setMyProperty(xxx);
em.getTransaction().begin();
em.getTransaction().commit();
[/snip]

And even though I didn't call persist, changes are updated in database. 
WHY???

Thanks B.