Brian Smith has posted a very interesting blog article about synchronizing users in Project Server 2010 versus Project Server 2013. He says there are behavior differences between Project Server 2010 and Project Server 2013 regarding user synchronization to project sites. I quote, "One key part of this change should be taken into account when migrating – as there is one 2010 setting that no longer has UI to change it – and if it is disabled before migration it cannot be turned on again in 2013."
Brian goes into details of that setting and provides a workaround after describing how the settings and behavior have evolved.
For anyone migrating from Project Server 2010 to Project Server 2013, this is a must read.