There's really no correlation between mondo size and normal shoe sizes. Hell... there's no comparison even between normal shoe sizes... Personally I have Adidas running shoes ranging from 42 to 44 2/3 (EURO size, but I guess that would go somewhere from 9 to 11 in US size), and they all fit perfectly, and what's even worse, they are all exactly same size! And they are all from same company, so you can imagine how these numbers go between different companies, and then imagine how they would apply to something totally different, like ski boots.
Otherwise as quite few stated already, race boots are smaller. My skiing boots (Fischer WC Pro 130 95mm) are 27.5. If I would be racing, I would move down to 27, considering current 27.5 is damn tight already. But for racing you need a bit extra "grip", so tighter and smaller boot gives you that extra feel needed to transfer your power and "feel" directly to ski. On the other side, my working boots are 28.5. They might be a little bit too big, but I can still ski down any of WC courses without problems, without feeling my leg is floating in boot etc. But once you are used to tight race boots, 1 number bigger boots, just don't feel right. But then again, there's really no way, I would be standing in race boots on hill for few hours. Too cold, too tight.
Considering majority of people don't ski like racers do, it's normal they don't have their equipment dialed in like racers do. First of all, my 27.5 boots needed whole bunch of work, before I was able to ski for 4 or 5 hours in them, without feeling I would be in middle of inquisition torture. Most people, don't have tools, and/or service and even money to do this kind of work. Just imagine how much would it cost, if you would need 10 or 15 visits to boot fitter who would be grinding your boots bit by bit after every new ski day, when your boots still wouldn't work perfectly. Not to mention, that a whole lot of people, hardly get 10 to 15 ski days in a season ;) To avoid this, you simply get a bit bigger boot, and you don't have those problems. And like I said, most of people don't ski so aggressively as racers do, so there's no need for such tight fits, and a bit bigger boots do just fine.