My GWF and my Polar 55 HRM usually read about the same ...
That said, there are a few exercises where they are different, and these seem to be things like:
* biking. The GWF is always on the low side relative to my HRM.
* strength training that's relatively steady. The GWF is usually higher. STS meso 3, the GWF is slightly lower for Legs.
* Cardio Coach interval work on the elliptical. Usually these meet after an hour, but early on, the GWF is a little lower.
On my body, anyway, the GWF tends to be a little lower when I'm doing really 'spiky' short/intense interval-oriented work, or biking (where the motion is supplemented by the bike instead of just being my body moving), or horse riding (same thing). I have noticed big differences for biking, but nothing as high as what you're reporting for a Cathe workout.
Hope that helps - these things are all individual. I tend to go with the GWF or a HRM, myself, and use the Workout Manager and other fixed numbers for cardio estimates as being a relative measure of intensity- a good relative estimate for what the workout will do compared to others, but not a number that will match what every given person will do on every given day. I rarely burn the same thing twice in the same workout