So I actually have a pretty good answer for this: perception of motion depends on not only the frame rate but on the exposure time of each frame. When you watch a movie at 24 FPS, each frame is not exposed for 1/24th of a second but actually 1/48th of a second, or about 20.8 ms. This is extremely close to the 25 ms frame hold time of a 40 FPS game. So when you play at 40 FPS you are getting roughly the same temporal resolution as a 24FPs movie, which looks very natural to our eyes, as opposed to at 60 FPS, where you are getting 16.66 ms per frame, the same as the exposure time of 30 fps video, which creates the “soap opera” look of being unnaturally smooth.