Probably not news to you:
That's because after they are split, they are each in a rectangle called an "Event".
You can't move both the start and end of each rectangle (Event) in one drag.
It doesn't make sense to move both sides at once. The way to visualize how the drags are working is try it all out on a single Event first, then test it on more.
For example: Get a single Event, try all the drag actions, then you'll see what it's doing in that case.
Then add a second event, select them both and do the drags again. S1 is totally consistent with itself in the way it handles those drag actions, and it makes sense that one drag is working on the starting (left) edge of the Events, the other drag is working on the end edge of the Events. (Or you could be using the drag location that will move the seam of both events at once, but it still won't do what you're trying to do.)
If you have a single rectangle (Event), you can't adjust both the start and the end edges of the rectangle, as that would simply be a move. (If you move the left edge right 2" and the right edge 2", the whole rectangle would move 2".)
Far easier to show, but if you mess with it all with a single Event, the behavior is easy to visualize.
Side note: There is more complexity than outlined above, because the behavior can be different depending on WHERE you drag the seams, in terms of top to bottom.
I hope this helps.