Tiers indicate what books mean to me; how much I'd recommend reading them. Rating is the actual quality of a book (given its context and time period).
I've read many more books of course, but they're not on this list. First off, there's no fiction on this tier list. I read very little of it, certainly not enough to have an interesting list of them.
Second, the list only contains books that could be considered a "must read" by at least some people. So no super-specific books like "ASP.NET 4 Internals" or "HTML 3.2 in 24 Hours" on here either.
Third, I don't like reading much (even though I do like to have read). So I tend to research what the classics and recommended books are before diving in at all. Therefore no fringe books are likely to appear on my list.
Lastly, I can easily drop a book if I dislike it; and not waste any further time on it. So books I quickly stopped reading (so especially low rated books) don't appear on my tier list because I dropped them before giving them a good rating.