Garmin data vs PT data

I am interested in hearing thoughts between Garmin's readings and from the Powertap headunit.

I did a 2 hour trainer ride today, Garmin gave 31.xx Powertap gave 36.4x
I noticed my 310 was set to use 2100 for the wheel and not the standard value,  but it should not have made up for this big of a difference.  

I also notice my Average Power is different, I believe Garmin said 185ish for the entire ride where the PT unit said 205.

I have the Garmin Cadence/speed sensor and a 2.4 SL+ setup.  

Any thoughts, bear possibly?