Hot Potatoes exercises do not function like a "test" in the way that you and your students appear to expect.
Please explain to your students that Hot Potatoes are learning exercises, in which they have to keep going till the find a correct response to each question. Only then will their results be sent back to Moodle and stored in the grade book.
If your students answers each question correctly first time, they will get 100%, but the more attempts they have at each question, and the more they make use of the hints and clues that are available, then the more their score wil drop.
Here is a tutorial on how to make JQuiz behave like a traditional test: