Pretty sure you misread the text here. This doesn't actually give you a higher AC. It just adds one number that you also dodge on on top of your current AC.

So, in the example in the image with an AC of 14 and if you rolled a 4 you would still have an AC of 14. So if someone rolled a 16 they would hit you. If they rolled a 22, they hit. 24? That's a hit. Only the specific number of 23 would count as a miss against your AC.

The funny thing is, this item would be all but useless for a character with high AC, since you would lamost always roll over what someone was going to realistically be hitting you often with.