I had a similar experience too, but I'm not positive whether it's based entirely on perception (which is certainly possible) or some kind of bug in the demo. It would be nice to save individual Ogg files at particular points on the continuum and try more of a blind test to see how much influence there is from what we've just heard before. (I guess I could generate those myself easily enough with sox or something...)
Wow, I experimented with it more and it's a matter of context, just as you and other people in this thread have described. That's really striking. Thanks for confirming that it's not a bug.
This is maybe an even more extreme phenomenon (from the same subreddit that was apparently involved in making the Laurel/Yanny thing go viral this past week):