One doesn't normally buy a 20 that is on a 12 frame. You can get them, but they are usually sub-gauge sets. No need to buy a 28 on a 20 frame if you don't want to. It has nothing to do with pride or skill. It does have a lot to do with having a good gun for the work it is doing, and a 28 balanced and of the right weight comes close to perfect for a Ruffed Grouse shotgun. In addition I've taken Pheasants cleanly out to around 35 yards with the Ithaca 37 pictured which is a 28 on a 28 frame. If one appreciates quick handling, than a balanced 28 on a 28 frame is not a bad place to be. The price of shells are mostly inconsequential when it comes to hunting, be it either rifles or shotguns.
