Hi,
this is because the Existence Test is configured in outbound sync rule but logically evaluated at the end of an inbound sync.
So since you have a connector to the datasource Attribut the DRE is still evaluated:
See:
https://technet.microsoft.com/en-us/library/ff608269%28v=ws.10%29.aspx
which states:
As a result, the actual existence test cannot be applied during the
outbound synchronization phase. For example, if your outbound
synchronization rule called Fabrikam Outbound Synchronization Rule that
you use to manage your Active Directory resources that have existence
test flow mappings configured, these flow mappings cannot be evaluated
when the outbound synchronization rule is applied to a resource.
In other words, while logically configured in an outbound
synchronization rule, existence test flow mappings belong technically to
an inbound operation. In our example, the actual existence test is
performed during the inbound synchronization phase of a synchronization
run on the related Active Directory management agent (MA).
-Peter