The GRE is adaptive, so how is it scored?

The GRE is adaptive between sections, but not within. What this means is that the questions do not change within the section depending on whether you answer them correctly. You can think of it this way – each section is static. Your performance on the first section will determine whether you get an easy section or a difficult section. The easy section is static and the difficult section is static. Again, this means the questions in the section do not change. You could miss the first ten and question 11 will still be question 11. You could work backwards from the last question, nailing all of them, and question 11 is still question 11. You can read more about this here: http://magoosh.com/gre/2012/is-the-revised-gre-adaptive/

Within each section, all questions contribute equally to your raw score, which is the number of questions you answered correctly. Your raw score is then converted to a scaled score through equating. Here's what ETS has to say about "equating": "The equating process accounts for minor variations in difficulty among the different test editions as well as the differences in difficulty introduced by the section-level adaptation. Thus a given scaled score for a particular measure reflects the same level of performance regardless of which second section was selected and when the test was taken."