Why would experimental Mach #s in a wind tunnel be higher that Mach #s predicted by theory (inviscid) for the same nozzle geometry

The figure below shows the mach numbers across a supersonice convergent-divergent nozzle.

Since the experimental Mach numbers (calculated from measured pressures along the walls of the nozzle) take into account viscous forces, shouldnt their mach numbers be lower than that of the theoretical inviscid predictions? Similar to the trend of the CFD (viscous) calculations?