Very interesting!
I may have missed this, but what was your body position during the test? I've always thought that a backpack could make a difference to a rider in a full tuck (flat back).
That was my thinking, so I was in the drops relaxed not straining for flat back but about 15 degrees up from flat. I wanted to test "commuting style" so I didn't go for a flat back or full downhill tuck. Riding on the hoods would be more normal for commuting though so I think I'll do that for the rest of the coast-down tests.