You may be confusing credits with percentage.
A mark of 50 credits means your submission was satisfactory. It does NOT mean you got everything right, but rather that you are on the right track, and can focus on ironing out the details, and fixing the mistakes I pointed out in your assignment.
A mark of 30 credits means that your assignment had some serious defects - more than just, for example, getting some of the tableau or Gentzen rules wrong, or struggling with the proofs in Question 2, or not understanding BDDs. You need to understand what went wrong and how to fix it, else the same will happen in the next two assignments and the exam.
A mark of 20 credits means you are way off track.
A mark of 10 credits means I could find no merit in your submission.
Students who submitted identical assignments received half the credits (e.g. 10 instead of 20).