Ribble 725 hardtail. With the shim it's 30.9 like stated in the specs, without its 33.0mm which is weird.
I contacted their customer service and they asked for a photo of the shim wtf.
I think this is fairly common. I guess it makes sense to use bigger diameter tubes in some cases (for stiffness etc) but use a sleeve to keep to normal seatpost sizes.
What’s the frame?