Joshua,
The exams do NOT increase in difficulty. Each exam is "manually curated" by our staff so that each exam is a realistic representation of what you can expect on the real exam. (Generally, the feedback is, however, that the questions and answers on the real exam are "more vague".)
And exam 8 is actually "the easiest". It consists of 100% of ITTO questions and so it is entirely memory-based questions and is not really what you will see on the exam. We have it because so many students like to study the ITTOs even though we don't recommend doing that anymore. Go figure...