You're right about question 2, my apologies. There indeed was a bug in the test data for that question (which we have fixed now). However, the rest of the questions given in the test work properly.

The reason different test takers get (somewhat) different question sets is because our algorithm alters the difficulty of the questions based on the test taker's performance. I.e. if you did the first few questions well, you'd start getting progressively tougher questions. On the other hand, if you do badly, you'll be given successively easier questions.