I don't know why for sure, but if I had to guess it would be that they limit the framerate in a way that's just course enough to lock to 31 instead of 30. Like, if you have vsync on at 60hz and you only update every 2 frames you would have a solid 30FPS, assuming you weren't limited in some way. But they may be limiting it in a 33ms per frame instead of 2 frames per refresh, you would get a different result, because 30 fps is 33.3333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333... ms per frame. So it's probably math that's the problem.
Also, 31 fps is only a problem because of 60hz displays cannot evenly display 31 fps. You have that extra frame, so there is a frame every 2 refreshes but that extra frame isn't just there by itself, you have 3 frames in a row that update at 16 ms, so 10% of the frames rendered per second have less persistence than the other 90%. If we had 31hz (or 62hz, or 124hz, or 93hz) monitors it wouldn't be a problem. Actually, it might still be a problem because xbox one probably doest support a locked 31hz refresh rate.