AMD brought this test to our attention, and the company actually sheds a bit more light on what this benchmark does than Valve's official release notes do. According to AMD, this test uses a technique called dynamic fidelity to adjust image quality to the level needed to maintain a high enough frame rate for a good VR experience.
Going by that description, the rather vague graph in Valve's report seems to tell us how much time cards can spend at various image quality levels. Unsurprisingly, then, the GeForce GTX 980 Ti's flat line at "Very High" means it didn't have to adjust image quality at all, while the GeForce GTX 970's more variable graph might indicate that more on-the-fly graphics settings changes were happening behind the scenes to keep frame times down.