Availability Groups Failover Readinness "No data loss" and rollback of nonqualified transactions
This doesn't count as data loss in this context. 

From the documentation (emphasis mine):


  A manual failover causes a synchronized secondary replica to
  transition to the primary role after a database administrator issues a
  manual-failover command on the server instance that hosts the target
  secondary replica. To support manual failover, the secondary replica
  and the current primary replica must both be configured for
  synchronous-commit mode, if any. Every secondary database on the
  availability replica must be joined to the availability group and
  synchronized with its corresponding primary database (that is, the
  secondary replica must be synchronized). This guarantees that every
  transaction that was committed on a former primary database has also
  been committed on the new primary database. Therefore, the new primary
  databases are identical to the old primary databases.


Any transaction in progress while the failover occurs is going to be rolled back and needs to be handled with re-try logic on the application side. You can't have a transaction start on one node, trigger a failover, and then have it finish running on a secondary node - though that would be pretty cool, admittedly. 

The synchronous replication just guarantees both nodes are in the same state before/after the failover, otherwise the former primary node could have writes that the now-primary node does not, causing inconsistent data betwixt the two.

Check out the very good documentation for more in-depth information about what really goes on, and what you're actually getting out of the AG feature set.