Why Does the Brain Lie About it?
Change blindness is usually related to something called inattention blindness. If you tried to process everything in your visual spectrum you would go insane, so your mind picks and chooses what to focus on. If Carlton grows a mustache while your brain isn't paying attention, when you look back at Carlton, your brain tells you he's had the 'stache all along.
It's like your brain is sitting in class, staring out the window at a cloud that sort of looks like a boob. When you call on your brain it does the same thing you do when a teacher calls on you in those circ*mstances: Start bulls**tting. It doesn't really know what Carlton looked like a second ago, but it's not going to tell you that. Since it has no visual memory of the image, it just tells you it's always looked the same. Even when that's a lie.
Where it Really Gets Weird...
What's truly amazing is just how often your brain isn't paying attention. Scientists decided to take the idea to a ridiculous extreme. They ran experiments where they'd have a guy manning the counter at an office serving students, while another guy was hidden below the counter. A student would walk up and request a form, and the guy would say sure and duck down behind the counter to get it.
But then second guy, the one who had been hiding, would pop up and say, "ah, here it is." This second guy would look completely different, and would be wearing completely different colored clothing, and most of the students would not freaking notice it was a different guy than the one they had been talking to five seconds ago.
Here is a video of such an experiment. Far creepier is the bit magician Derren Brown does where he'll approach a stranger on the street, ask for directions, and in mid-sentence have somebody walk past carrying a large object. While the object is disrupting the view for half a second, he'll swap out another guy who looks and sounds nothing like him--and the stranger will carry on the conversation with the second man as if nothing had happened.