Why is the floor in PeriopSim not correct?

You setup the floor level in Windows MR but when you launch PeriopSim is is mis-matched.

One possible reason is the mode in which you setup the system. Assuming you set the floor height correctly Tip on Floor Levelling

When setting up the system you will be asked what mode to set it up into.WMRSetupOptions_0 (1) 

Based on you select and where the headset is at the time you may confuse the system.

If you say to Sitting but then stand up to go into PeriopSim as it is a standing experience this can cause floor to adjusted as the headset is above the normal sitting level.

Less likely is 

If you say to use Standing but the headset is on the desk or sitting level for calibration