The period of the pulsating pressure is 50 ms. When you simulate with a 10 ms time step, you are putting just 5 points on a single cycle of a sinusoidal curve. That is a very poor representation of the curve. The recommendation is a minimum of 20 points per cycle, therefore you would have a time step of 2.5 ms.

How are you initializing the simulation? You could run a Steady State simulation and converge on a flow at the average pressure of 1380 kPa. Are you doing this? Then from this steady state flow, begin the Transient simulation and introduce the pulsations. How many seconds are you simulating? If your end time in the Transient is 2.5 seconds, that is 1000 time steps.

The oscillating pressure goes from peak to valley in 25 ms therefore, the forces on the walls are going to be different at the maximum pressure compared with the minimum pressure. Make a plot at two different times, one when the pressure is maximum and another plot when the pressure is minimum.