Since the range is 65 to 75, there are only 11 choices. So the left part chooses a number from 0 to 10, and then we add the $low end of the range back to turn it into 65 to 75.
Update: as one popular signature goes: Figure out how you'd do it by hand, then program the computer. When you're playing against another person, you don't just guess a random number between 1 and 10 each time, do you? If you did, people would stare at you in puzzlement when you guess 5 they say "high" and then you guess 3. So think about how you do your guessing, and make the computer do it the same way.