Well, they re-wrote enough of the engine (Todd Howard says MOST of it) that they renamed the engine and the content creation tools. When you start rewriting core modules of the engine, that isn't really polishing. At least that's how Todd Howard makes it sound. The rendering engine comes to mind, which he says they changed entirely. That's a pretty big change, and they also said they changed the gameplay engine entirely. That could mean a lot of different things, but is also a core element of the experience.
But then again, if they can do that and "get away" with it. It's understandable. Oblivion looked immensely better than morrowind, but to be honest even if there's a greater gap from morrowind to oblivion than there's from Oblivion to Skyrim, graphics haven't jumped all that far like when you compare former two, still animation is gorgeous. I also hope they will have some cool quests like in Oblivion (to go inside people mind), cool places (Shivering Isles) and 100+ hours of gameplay for dedicated folks. They've said 300+ but I can only hope, Bethesda is currently amongst top 3 game devs (alongside Bioware and Capcom (Atlus for smaller ones)) and unlike Capcom and Bioware they didn't betray the genre or platform like formentioned (DMC5/RES5 , ME2/DA2)... awful much.