Over on ToolGuyd, there's a discussion in comments as to how the wrench slipped with the non-Wright fasteners.
I'm thinking there are two possibilities:
1. The wrench rounded off the corners and then slipped.
2. The wrench spread a little and slipped, and rounded off the corners in the process.
A reader suggested that you examine the width of the wrench openings to gauge whether they're spreading -
http://toolguyd.com/anti-slip-wrenches/#comment-197868 .
I don't think that this should be done with calipers, but it can be done from directly above, if you have a tripod, mount, or second set of hands to hold the camera above and parallel to the wrench.
Grade 5 fasteners, let alone Grade 8, shouldn't round easily.
Hardness data:
http://www.fastenal.com/content/feds/pdf/Mechanical Properties of Inch Fasteners.pdf