Everything I've seen indicates that the composite video output on the D925, whether though the RCA jack or the SCART connection, is the chroma and luma of the s-video output combined - that is, the signal always goes through the digital processing circuits, getting filtered in digital RGB and converted into luma and chroma along the way.
Some users have suggested a mod to pick up the "clean" composite signal before it enters the frame store, but no-one I know of has sucessfully implemented this yet.
Perhaps the difference you are seeing is due to your SCART cable being better shielded than your RCA cable?
(Although, I do remember Laserman talking about the V800, saying the BNC connection output a pure composite signal while the RCA is the combined s-video. So this kind of thing is not totally unheard of...)