Hi,
As you know Zpv is computed from power and voltage, Zpi power and current, Zvi voltage and current. HFSS computes the voltage by electric field integration across a line from GND to signal. Current is from magnetic field and the power from the poynting vector. The closest value is the computed from p and v. Why? The power is easily computed from E and H*. Voltage integration of E. That means you have defined the impedance line properly and that the current is not being accuratly computed. You must check in the manual how is the current computed and maybe it is no possible to compute the correct value for CPWs structures.
I hope it was helpfull!!
clk