Attenuation constant in a coupled line simulation

I'm using HFSS for a specific coupled CPW line simulation.

Driven modal are used. Two waveports are places at both ends of the coupled CPW line and two modes (even mode and odd mode) are excited at each waveport. However, at two ends of the coupled line, the simulated gamma shows different results.

Has anyone observed such inconsistancy in HFSS coupled line?

Here is a snapshot of my simulation result.

Thanks.