Could someone intelligent please explain something to me.

One of my cards (290) has the following stats:

HRA: 870 kh/s
Rejects: 3.6%
WU: 811.7/m
HW: 0

So by my calculations the efficiency is 101.9%

efficiency = WU * 65536 / 60 / 1000 / HRA
(*100 to get res. as %)

How can it be that high with 3.6% rejects?