Limpopo has it right, IMHO. The ANR systems reduce the apparent noise, but only below 1000Hz, so that much of the real noise is not electronically attenuated. The higher frequencies can only be blocked in the conventional way by a good sealing earcup. If the Bose does not have that good earcup, it will not protect as well as one that does.
Limpopo, do you know where there is any data? (Great gray green greasy, as Rudyard told it...)