I aint know a thing how SA works, but as far as i understand, FPS represents speed at which SA works on your PC.
When you have 70 FPS, between frames there are 14 ms. So, your PC works only with frame 1 (0 ms) and frame 2 (14 ms). It has speed (and velocity), acceleration and everything else at point 1 (frame 1) and it is doing "math" to find out velocity-accelertion-else at point 2 (frame 2).
Now, if you have 140 FPS, there are 7 ms between frames. If we take same process we had with 70 FPS, we will have frame 1 (0 ms), frame 2 (7 ms) and frame 3 (14 ms). And do you really think that velocity-acceleration-else after 14 ms with 70 and 140 FPS will be the same? If there is 7 ms delay, between 0 ms and 14 ms there will be additional "math step". The lower ms delay we will take, the better representation we will get of our "run-up". When we ride on some plain surface, difference will be 0. But what if run-up goes up and down, has many "curbs" and everything else? As i can see it, this is why higher FPS helps you to "avoid" curbs on run-up (because there is more math steps and representation of run-up is more smooth).
Now, we know that with higher FPS you stops faster. Why? Because game has only data (speed-acceleration-else) only on those exact "frames". At 0 ms game knows you touch the ground, so it decreases your speed by, lets say, 1%. After 14 ms there is next frame, and again game gives you 1% decrease because you are on the ground. But what if delay was 7 ms? Between 0 ms and 14 ms there will be additional frame with additional 1% decrease. And the same goes with grinds.
Now, lets get a situation when our speed and ms delay is such that some frame exists before bump, and next frame exists after bump. When we are on frame "before bump", game has our "speed-acceleration-else". Then it does "math", it has our current coordinates and knows there must be a "bump". So, it changes our direction, but only at frame "after the bump". So, if framerate will be low as fuck, i guess it will looks like you went through the bump, and only after that you started to fly upward (or fall down from bike).
And the bigger FPS we will have, the closer to actual bump we will start to ascend. So, bigger FPS "describes" actual run-up in a better, smoother way.
OR everything above is bullshit and has nothing to do with how SA actually works. I do not know if game do math only on "frames", i do not know how it works with run-up between frames (on frames it can calculate your actual position if it knows map, but what is happening between frames it can only take into consideration on the next frame).