Chaos theory - just because you lay the asparagus perpendicular to the grates doesn't mean that one or two don't find their way into the fire when flipping or taking them off - at least in my experience.
I did the asparagus on the mat mainly as a test to see if I could get the supposed "grill marks" with a dry cook. Nada! (Oh, and because it had been sitting in my veggie bin in the fridge since going to the market last Wednesday and needed to be cooked!)