I have long suspected that any level 20 (ilevel 28/29) epics now in existence are grandfathered. Almost every realm shows the same items that are scaled below or up to 20 as ilevel 25/26. Until we see definitive proof of a level 20 looting a new Destiny, or "Classic" world epic I will continue to doubt some still drop with that scaling.
If you look at the Atlas Loot add-on some items won't link lower than specific levels. These may be their level "floors" for any new drops (which would explain why so many are level 22/24).
Another quirk I have noticed, (and haven't shared until now) is that some items will NOT show up on AH searches for level 20. Icemail Jerkin, Gloves of Holy Might, Robes of Insight and Stonerender Gauntlets (confirmed for me so far) will sometimes NOT show up if you set the AH search limit to 20. If you set the AH limit to 24 it WILL show these items (Requires Level 20).
I haven't been able to find any other epics than those four specific items but it might be worth checking on your realms' AH. There could be other ultra rare items seemingly missed because most people set the AH level limit to 20.
Note: You also have to click on the individual item to check its true level (The mouseover is incorrect on some items).