I agree with what Brent says above - questions which are easy for one test taker may be hard for another. But I think there are other factors at work, and I'd only take the position of questions in the OG as a kind of very rough estimate of their difficulty. I don't think, for example, that anyone could reasonably argue that Q220 in OG13 (PS section) is harder than Q199, even if Q220 comes later in the book. In fact, I can't really see how Q220 can even be a 500-level question, while I think Q199 is one that many 750-level test takers would find challenging.
The questions in OG13 come from all kinds of different periods in the GMAT's history; some OG questions are more than a decade old, and some are quite recent. The GMAT has gotten a lot harder over the last ten years - a 700-level question now is much harder, mathematically speaking, than a 700-level question from 20 years ago. I very often find the older questions in OG13 (questions like Q220) are placed too late in the book compared to their difficulty level for the average test taker, and the questions which have been recently added (questions like Q199) are often too early in the book.
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