An observation I had this summer: I commute with my gravel bike and had to put an old tire (Schwalbe G-One Bite) on my gravel bike instead of the usual tire (Specialized Pathfinder Pro). Rolling resistance says Pathfinder has 20.3W while the Schwalbe is around 25W. Still I was faster on the Schwalbe G-One Bite.
The reason is that it grips much better and I’m able to take curves faster - for every curve I save several seconds. It’s “slippery” gravel where going straight feels save, but taking turns one is never sure if a tire starts losing grip. There is a 5-min-segment where I’m up to 25s faster with slower, but better gripping tires. I’m sure I could learn to take faster curves with other tires too. 
Back to topic: I would love to have a table with a standardized number for “grip” of gravel und MTB tires.