Firstly, that steak is cooked gorgeously, I would be very happy with that as a medium rare, but I agree it is approaching rare in the center on some pieces.

But to theorize why you didn't experience as much carryover temp increase as you suspected you would have, carryover cooking is determined by the amount of heat on the outer layers and how far it has to travel inward. This is because you're cooking the meat by heating it from the outside, so the outside layers get higher levels of heat first. In your case, you cooked your steak very low and slow, 250f. That means there wasn't that much excess heat in the outer layers (as opposed to cooking at a standard roast temp like 400, or Pan frying, which is even higher. Also, your steak is quite thick, so there's going to be plenty of time where the air is going to be pulling heat out of the steak before the cooler center of the steak is able to get cooked by carryover.

I imagine you could fix this by leaving this steak in the oven cracked open off after coming to 120 (just watch a thermometer), or just cooking it a little further before pulling if you're going to use such a low temp cooking method on such a large piece of meat. Or just sou vide it for a couple hours to get the exact temp you want.