How does a consumer microphone achieve a sensitivity of -20 dB?
Probably an amplifier (gain would be around 10 or 15, no big deal). 

There are 'silicon microphones' with similar sensitivity (-18dB when the internal amplifier is set for +20dB gain) such as this one. 

SNR is the more important spec (you can always add gain), and the 'consumer mic' is pretty lousy (58dB) vs. 74dB for your electret mic and 63dB for the one I linked to.