Why is my mbpro mic better than my usb

I bought a decent plantronics usb microphone - but people keep on telling me the mac mic is better, when they talk to me online. Does anyone know why this is - been through a bunch of usb mics and this is usually the case. It's annoying because it means i have to speak louder for them to hear me, or can't close the mbpro (I'm usually connected to an external monitor).

Anyone recommend any microphones?