Why do women think it's okay to say a 12 inch penis is average

My exgf told me that "some guys have bigger alphas, and it's cool you're an alpha male, but some guys have 12" penises" and I was like "no, most have 5-6" penises" and all of my guy friends are like "how the fuck do people think that's normal? Most men are like 5.5" or less". So I'm curious, is it an even 50/50 split of people with this false belief, or what the fuck