But your post hasn't offered an explanation. And I don't, in fact, look at that image and think that the two squares should look like the same color.
A and B are in fact different colors, for a value of "in fact" which takes into account that the picture is a picture of something - a checkerboard. My visual system makes the correct inference, conditioned on the assumption that I'm looking at a checkerboard.
EDIT: what I should say is that I'm still surprised, knowing what I know about my visual system and how it works, when you tell me that the pixels have the same RGB values. But that's not a "reality is weird" surprise, it's more like the surprise of learning some interesting bit of trivia.
To really be totally unsurprised, I'd have to enhance not just my knowledge of the visual system, but my visual system - to include an RGB calibration system.
EDIT: Oh, okay, I read your edit and that makes much more sense. I agree that it may be difficult to get to the point of being unsurprised. Getting there isn't obvious. You know you are there when you are unsurprised by the illusion. Once Reality is unsurprising and obvious, you are there.
I feel like I have lost the point of this conversation. What, in the following, do you disagree with?
Today I looked at the above illusion and thought, "Why do I keep thinking A and B are different colors? Obviously, something is wrong with how I am thinking about colors." I am being stupid when my I look at this illusion and I interpret the data in such a way to determine distinct colors. My expectations of reality and the information being transmitted and received are not lining up. If they were, the illusion wouldn't be an illusion.
The number 2 is prime; the number 6 is not. What about the number 1? Prime is defined as a natural number with exactly two divisors. 1 is an illusionary prime if you use a poor definition such as, "Prime is a number that is only divisible by itself and 1." Building on these bad assumptions could result in all sorts of weird results much like dividing by 0 can make it look like 2 = 1. What a tricky illusion!
An optical illusion is only bizarre if you are making a bad assumption about how your visual system is supposed to be working. It is a flaw in the Map, not the Territory. I should stop thinking that the visual system is reporting RGB style colors. It isn't. And, now that I know this, I am suddenly curious about what it is reporting. I have dropped a bad belief and am looking for a replacement. In this case, my visual system is distinguishing between something else entirely. Now that I have the right answer, this optical illusion should become as uninteresting as questioning whether 1 is prime. It should stop being weird, bizarre, and incredible. It merely highlights an obvious reality.
Addendum: This post was edited to fix a few problems and errors. If you are at all interested in more details behind the illusion presented here, there are a handful of excellent comments below.