If an experiment shows that there is no difference, what is the explanation?
My non-scientific guess would be that a stopped propellor blade converts all the energy into turbulence. While a moving blade provides a little less full-on face to the airflow and thus generates less turbulence, but at the same time converts some energy into heat due to the internal friction of the engine.
Of course there will be differences in the exact energy conversion, but as we've seen here, the difference is negligable in a typical spamcan.