PSI - Statusing Web Service - Results not as expected - status

Question

I'm trying to update Status information on assignments via Statusing Web Service (PSI). Problem is, that the results are not as expected. I'll try to explain what I'm doing in detail:
Two cases:
1) An assignment for the resource exists on specified tasks. I want to report work actuals (update status).
2) There is no assignment for the resource on specified tasks. I want to create the assignment and report work actuals.
I have one task in my project (Auto scheduled, Fixed work). Resource availability of all resources is set to 100%. They all have the same calendar.
Name: Task 31 - Fixed Work
Duration: 12,5 days?
Start: Thu 14.03.13
Finish: Tue 02.04.13
Resource Names: Resource 1
Work: 100 hrs
First I execute an UpdateStatus with the following ChangeXML
<Changes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Proj ID="a8a601ce-f3ab-4c01-97ce-fecdad2359d9">
<Assn ID="d7273a28-c038-486b-b997-cdb2450ceef5" ResID="8a164257-7960-4b76-9506-ccd0efabdb72">
<Change PID="251658250">900000</Change>
</Assn>
</Proj>
</Changes>
Then I call a SubmitStatusForResource
client.SubmitStatusForResource(new Guid("8a164257-7960-4b76-9506-ccd0efabdb72"), null, "auto submit PSIStatusingGateway");
The following entry pops up in approval center (which is as I expected it):
Status Update; Task 31; Task update; Resource 1; 3/20/2012; 15h; 15%;
85h
Update in Project (still looks fine):
Task Name: Task 31 - Fixed Work
Duration: 12,5 days?
Start: Thu 14.03.13
Finish: Tue 02.04.13
Resource Names: Resource 1
Work: 100 hrs
Actual Work: 15 hrs
Remaining Work: 85 hrs
Then second case is executed: First I create a new assignment...
client.CreateNewAssignmentWithWork(
sName: Task 31 - Fixed Work,
projGuid: "a8a601ce-f3ab-4c01-97ce-fecdad2359d9",
taskGuid: "024d7b61-858b-40bb-ade3-009d7d821b3f",
assnGuid: "e3451938-36a5-4df3-87b1-0eb4b25a1dab",
sumTaskGuid: Guid.Empty,
dtStart: 14.03.2013 08:00:00,
dtFinish: 02.04.2013 15:36:00,
actWork: 900000,
fMilestone: false,
fAddToTimesheet: false,
fSubmit: false,
sComment: "auto commit...");
Then I call the UpdateStatus again:
<Changes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Proj ID="a8a601ce-f3ab-4c01-97ce-fecdad2359d9">
<Assn ID="e3451938-36a5-4df3-87b1-0eb4b25a1dab" ResID="c59ad8e2-7533-47bd-baa5-f5b03c3c43d6">
<Change PID="251658250">900000</Change>
</Assn>
</Proj>
</Changes>
And finally the SubmitStatusForResource again
client.SubmitStatusForResource(new Guid("c59ad8e2-7533-47bd-baa5-f5b03c3c43d6"), null, "auto submit PSIStatusingGateway");
This creates the following entry in approval center:
Status Update; Task 31 - Fixed Work; New reassignment request;
Resource 2; 3/20/2012; 15h; 100%; 0h
I accept it and update my project:
Name: Task 31 - Fixed Work
Duration: 6,76 days?
Start: Thu 14.03.13
Finish: Mon 25.03.13
Resource Names: Resource 1;Resource 2
Work: 69,05 hrs
Actual Work: 30 hrs
Remaining Work: 39,05 hrs
And I really don't get, why the new work would be 69,05 hours. The results I expected would have been:
Name: Task 31 - Fixed Work
Duration: 6,76 days?
Start: Thu 14.03.13
Finish: Mon 25.03.13
Resource Names: Resource 1;Resource 2
Work: 65 hrs
Actual Work: 30 hrs
Remaining Work: 35 hrs
I've spend quite a bunch of time, trying to find out, how to update the values to get the results that I want. I really would appreciate some help. This makes me want to rip my hair out!
Thanks in advance
PS: Forgot to say that I'm working with MS Project Server 2010 and MS Project Professional 2010