Please have a look at the explanations here: StaMPS/matlab/ps_plot.m at master · dbekaert/StaMPS · GitHub
This is related to the reference value which can be set by StaMPS individually. If no point is selected, StaMPS takes the average value of all PS instead. It is discussed and explained here: How can I define reference area in SNAP - #6 by falahfakhri
So when you select a suitable point, your time series will start at 0.