That's good info because it is counter-intuitive to what I'd expect. In my (feeble) mind I'd expect more powder would produce more unburnt powder since not all of it would have a chance to burn before the bullet left the case.Yes if you are getting sooty cases from blow back or unburned powder flakes. Check your data and see where you ate at load wise.
But I suppose it could be explained by increased powder volume decreases case volume increasing pressure a little faster thus burning more before the bullet leaves the case.