GMAT Question difficulty discrapency?

I really don't know what to make of the GMAT practice questions I've taken so far. They are all over the place. Here are how I would rate them for all of the sources that I took practice questions from 1-10 (10 is the most difficult):

1) First GMATPrep- first one I took, before I did any studying. I remember thinking that it was challenging but easier than I thought it would be. I don't really remember specifics well though. It was a while ago. I got a 610. So I'd give it a 5 in what I felt its difficulty is.

2) Kaplan (pen and pencil diagnostic)- I thought it was pretty easy and when I went over the answers and felt that I should have done even better. I got a 660. I'd give it a 3.

3) Second GMATPrep- I'd give the math and verbal a 7. The verbal is difficult to gauge since it was pretty late at the time, and I think that effected my score.

4) Official GMAT Guide Diagnostic- Math- 7.5, I thought it was a little tougher than the second GMATPrep. Verbal, I'd give it a 5.5, would've been a 5 if not for one reading comp passage about science (I got 3 of the 5 reading comp questions wrong on this damn passage). I did decently. Above average on each one.

5) Official GMAT Guide Math questions in the problem solving section- This is the biggest descrepancy I've found so far. I haven't checked the answers yet, so maybe it wasn't as easy as I thought but I'd seriously have to give it something like a 1.5. How is it that these are so much easier than the diagnostic questions?

6) EZ Solutions advanced math- I bought this book at a time when I was feeling good about myself. I remember people here or somewhere else were touting how difficult it was. I don't know if it's because I just went through less than 3 chapters (I'm in the middle of the 3rd), but I don't find it too difficult at all. The 3rd chapter got decently more difficult, but for a while the only mistakes I was making were silly ones that I should have gotten right. Because the 3rd chapter got more difficult I'll give it a 3.5. So far I'm somewhat disappointed.

7) Kaplan CAP 1- This is the Big Kahuna. Everyone talks about these tests as being harder than the the rest. You know what? IT'S TRUE! The first couple of questions I found easy and thought "this isn't that big of a deal". However at some point the tests got tougher and I ended up spending crazy amounts of time and at one point I was running out of time and just began guessing and missed 4 questions. The English started off easy too, but the stupid scientific reading comp passages, which I see no cure for, killed me. How do I answer questions about passages that I can't understand? I suck at science. I don't understand these scientific terms. So I ended up with even less time than for the quant. Difficulty is a 9. I scored a pathetic 510 and was shocked it was that high.

What do I make of this range of difficulties? I feel like I'm being jerked around into different directions. Especially the GMAT official guide. Are they trying to screw with you?