Because if the circlet is nigh sa'angreal, and Lanfear is the most powerful female channeler, she ought to have obliterated Rand and co. Unless:

a) she was holding back for some reason (Further clarification from Cyndane in AMoL would be nice if this were the case)
b) Rand's fat man is capable of drawing more OP when "forced"... like an elastic band (and we've never seen more than one angreal of this sort)

I mean you're an OP expert: was Rand even at his full strength then?

Either way, I hope we learn more of the bracelet. I remember from FoH, Moiraine saying she had a disliking towards it. Was it because she knew what it would compensate for in the future? What more is there to this nigh-sa'angreal than meets the eye?