Some things you should look at:
1. Check the RFC adapter logfile and the project log file. Change the settings on both the project and the adapter to make the logfiles more verbose. See if they provide cludes
2. Check the mapping file used by the RFC output adapter. In particular: are there any data type issues? the average calculation, for example, may change the datatype (eg an average across a set of integers will produce a float, etc)
3. Or could it be an opCode issue? I don't know how the RFC adapter handles opcodes. If you aren't familiar with OpCodes in SDS, see this post: http://scn.sap.com/docs/DOC-40207. The input stream is emitting all insert events, but the aggregation window will be emitting update events after the first one for each key.