So why a racquet that stiff with a poly strung that high bothers my arm less than a softer racquet at a lower tension is beyond me.
Some polys when strung high enough lose their resiliency and will continue to deform as you hit. Sort of like TCS polys at pretty much any tension. I have no idea if Black Widow would work like that or at what tension it would need to be strung to reach it's limit, but it's a possible explanation. When I played with the PDR I found it pretty comfortable with most of the strings I used, polys included, but I was stringing around 55lbs IIRC.