coursepress grade math is bizarre

coursepress grade math is bizarre
For example, if I answer one question correctly of 3 single-choice questions, the grade is 50%. Two correct responses gives 83%. Two incorrect responses gives 50%. Am I just missing some clever sophisticated setup here? Because it seems broken.

Here is a little table with a set of 3 single-choice questions:
right wrong unanswered total grade
0 0 3 3 0.00%
1 0 2 3 50.00%
2 0 1 3 83.00%
3 0 0 3 100.00%
0 1 2 3 33.00%
0 2 1 3 50.00%
0 3 0 3 50.00%
1 0 2 3 50.00%
1 1 1 3 67.00%
2 1 0 3 83.00%

Please fix this, or help me make sense of it.