So it's normal for some passes to take longer to finish than others because the tests write to the computer in different ways everytime?
Safer and easier to assume that some passes take longer or vary in time. What is important at the end of the day with memtest is find errors if there are any.
Here's the description for the tests:http://www.memtest86.com/tech.html#descri
Personally I just look if memtest completes a few passes without errors rather than the number of passes done.