New Pressure Treated boards are wider than existing ones

It could be, but not necessarily the new you think. The new is likely a new brand. Even if you bought boards from the same place, they could have a new supplier and have gotten rid of the old supplier.

We do have standardization for a reason, but there are plenty of cheaters in everything, wood is no different. You're lucky to get a skin-thin face on furniture-grade plywood from a big-box store and can only get real plywood from an actual lumber yard.

With all that said, also consider that the new stuff is literally wet. Many Mom & Pop places store their PT out in the rain. You may find a dimensional match with a month of drying or something closer that can work a little better...short of ripping them down and re-easing the cut edge.

That is because the new boards are still "wet" with the pressure treatment and are new. Once they dry out, in a few months, they will be the same size or close to it. Lay out the deck with the boards but don't screw them down yet. Let them dry out, flipping them over often. I let mine dry out for 12 weeks. Don't seal them yet either.