Reading a book in a few hours is dead easy but to get the grasp of it and understand the knowledge inside in a few hours is impossible. I'd rather read a book through the first time and understand everything rather then re-read a book 50 times in a few short hours and then finally be able to understand it.
Hm, I can't say that I don't understand the book just because I read fast. I get everything, every detail and story subplot. In fact, I can't read any slower. It would make me throwing the book into a corner because I can't stay focused on a single sentence for long.
However, I noticed that details don't stay as long in the longtime memory as they do for other readers. It fades faster. I always notice that in the Naruto board where some members have incredible memory on character details which I forgot years ago already, soon after reading the chapter.
But I don't see that as negative since I love re-reading a good story. When I like a book or a comic I will definitely read it again. If I like it again, I will read it a third or more times. That's just my style of reading. Literature which is trash to me will fade fast from my memory while good works will be re-read once in a while to enjoy them again (and with every time, more and more details stick in my brain forever). Similar to Katy I have books in shelf that I read 20 times and more and I enjoyed them every time, particularly because of the fact that I forgot on details and get surprised again.
Also, for the 1200+ manga on my read list. Don't forget my age. I read manga (and comics in general) for a little longer than most members of this forum.
You also have to take into account that MU's read lists are done by the title not by volume number. A large amount of the title on my lists are oneshots and short stories from one to ten volumes maximum. Very few series are above 15 volumes and even less in the range of Naruto and similar counts.
That means e.g. Inuyasha appears just as one title on my list but has in fact 56 volumes. Now, I have just as much and more one-volume manga on the list which have combined the same volume length but appear with 56 titles instead, as well as a dozens of oneshot manga which would (if you'd count 8 chapter per volume) add up to 448 titles for 56 volumes worth of read. Gotcha?
..especially character description). 
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. If it didn't then we're screwed then aren't we. lol.
. Or I can be arrogant and be #1