I would assume it's the muffling action of the smaller pipe - on the low notes, not the high-pitched whirring. My e-cutout is just before the gas tank, and I really don't hear the whirring sound anymore. I ran my EF with the DP stop at the firewall, got the whirring but overall the exhasut was loud as fuck and drown the whirring.
The backpressure idea wouldn't really fit, since less restriction after the turbine should always increase the overall pressure drop - meaning less manifold backpressure needed for the same turbine shaft speed.
One thing to note, Aero said his VGT Holset sounded a lot quieter, but more jet-like, at idle when the VGT was closed/restricted. I know that's partially due to the increased shaft speed, but might also be due to the extra muffling. I also know people who have used a quick spool valve say the same thing when they block off a volute.