This is a fantastic question that many people have. First, let's clear up some common misconceptions:
- The score is not directly based on a % correct.
- The score does not necessarily correspond directly to the number you answered correctly.
- If you got 20/40, that does not mean you should have a 150, for example.
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, a process that ETS explains on their website as:
"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."
What this means in practice is that the hypothetical 20/40 score may be anywhere from a 145 to a 155 without anyone finding this odd at all, due to the equating process. Many students assume the raw to equated score conversion will simply mean mapping the raw score onto this new scale, but now you should see that this is not quite so. This equating process, while a little confusing at first, helps explain why a student wants to answer questions right and get a harder section, even if this means missing more in the second section. You get "credit" for it in a way.
So it is unlikely that the grading algorithm of your mock test was completely off. The thing you are seeing is normal! :)