It's not the problem with your interface but it's the standard behavior of the soap sender adapter.
To get a better understanding of the issue and work around solution is clearly addressed in the below thread -
http://scn.sap.com/thread/1627368
Further reference :856597 - FAQ: XI 3.0 / PI 7.0/7.1/7.3 SOAP Adapter
Alternate Solution : 1459995 - Soap Sender Adapter HTTP 202 response for asynchronous call