A book has 10 short and 10 long chapters. Short chapters span 10 pages, and long chapters span 20 pages.
Why does the probability that you will pick a long or a short chapter differ between these strategies?
Strategy #1: Flip to a random page, back up to the start of that chapter, and start reading.
Strategy #2: Flip to a random page, go forward to the start of the next chapter, and start reading (and pick the first chapter if the page you pick lies within the last chapter).