You need to understand the difference between simulation steps and iteration steps. It looked like you ran simulation for 20 seconds. To simplify, let's assume your simulation step size is 1 second. When the simulation is run at time=1 second, the "For iterator subsystem" is executed 20 times. It itrated from 1 to 20 times. When the 20 iteration is done, the result is output to the Scope. At time=2 second, all these 20 iteration is done again and so is for t=3,4,5,... seconds.
So the Scope in the main modle always show 20. The scope inside the "For iterator subsystem" is also mostly 20 but it also shows the iterator goes back to 1.
XY Graph is different. It just shows the X and Y value pair. Because it is inside the "For Iterator subsystem", it probably shows all the value. But you will get the same graph even if you just ran the simulation for 1 or 2 seconds.