The reason is clear:When you use a 8 ohm compressiondriver with a sensitivity of 106dB/1w/1m and a woofer with a sensitivity of 103dB/1w/1m then you have to add resistors (attenuation) to make the efficiency of the compressiondriver also 103dB. In this case some power (equal to the power dissipated in the driver which can be up to 200W or more in PA applications) is lost in the resistors. When you use a 16ohm compressiondriver with a sensitivity of 106dB/1w/1m and a 8ohm woofer with a sensitivity of 103dB/1w/1m you don't have to add the resistors. The load on the amplifier decreases and this results in more power for the woofer and a increase in headroom.
Remember that compressiondriver are build for the use in PA applications and not hifi!
Best regards,
Walt (De Jong Soundsystems)