It's so hard to explain. Just do these so you can understand what I meant.

1. Make three frames. Write a big "1", "2", and "3" to each frame.
2. Make each frame 10 frames long.
3. Play it.
4. Now move the frames and make them like 15 frames long or 20 frames long. You will notice the timing has not changed.

I assume that the timeline did not "refresh" itself after the changes are made. It only refreshes itself when we draw something on the frames.
So I did that. I draw something and then undo on each frame. Now it shows the changes.

The best and fastest way to fix this is close the window and open it again. I think having a refresh button on the timeline is helpful.

Edit: You can also refresh the timeline by making one of the layers invisible and then back to visible.