Need Help,hfss & Si8000

Hi,

I simulated the microstrip of PCB with Hfss and SI8000,but the result is different.
PCB board high 0.175mm, FR4(Er=4.5), 50ohm trace width is 0.28mm with Polar SI8000 and 0.2mm with Hfss.
why?

thanks

HFSs sometimes needs lots of mesh for planar structures with a highe aspect ratio... start with some heavy duty seeding then a few adaptive passes...

This should improve.