Hah, I was thinking the same thing.
First time I did the fighters guild took me 12 hours. I started from out of the sewers and became the Master of the Guild. So just that questline alone (if you've already done it), could probably be done within 6-7 hours.
People usually tend to include their idle time, doing other stuff in game, doing alchemy, gathering plants, looking at the environment, looking what skills do, trying some random irrelevant things etc to the times they actually spent properly doing the quests. Plus all the slow stealth movement on places where it isn't actually needed, slow fights because of stupid items, bad usage of spells, using useless skills etc. Of course I did the 100% runthrough within ~24 hours because I had already done almost everything of those already before years ago and when I couldn't find some item required etc I just looked up Oblivion's wiki. When I first played Oblivion I also did a lot of idling, exploring etc but still the game was finished less than a week at least and then I was a very slow gamer.
http://forums.3drealms.com/vb/showthread.php?t=28776 even tho the video isn't up anymore, seems this guy did the whole 100% walkthrough within 15 hours in 2007.
I don't understand why you don't understand that it takes so little time to finish all the quests.