Let us suppose that you take a test with 5 questions. You answer three correctly, one incorrectly, and you leave one question unanswered.
Your percentages will be: 60% correct, 20% incorrect, 20% unanswered.
The number of users answering same questions:

Correctly 
Incorrectly 
No Answer 
Q1: (out of 30 users) 
5 
15 
10 
Q2: (out of 70 users) 
9 
11 
50 
Q3: (out of 20 users) 
2 
8 
10 
Q4: (out of 2 users) 
1 
1 
0 
Total: 
17 
35 
70 
Q1: 30 people had this question in their test and submitted the test including this question.
Q2: 70 people had this question in their test and submitted the test including this question.
Q3: 20 people had this question in their test and submitted the test including this question.
Q4: 2 people had this question in their test and submitted the test including this question.
Out of 30 people the correct answer for the question Q1 was given by 5 users;
Out of 30 people an incorrect answer for the question Q1 was given by 15 users;
Out of 30 people for the question Q1 10 users did not give any answer;
Out of 70 people the correct answer for the question Q2 was given by 9 users;
Out of 70 people the incorrect answer for the question Q2 was given by 11 users;
Out of 70 people for the question Q2 50 users did not give any answer;
...and so on.
The statistics is going to be the following:
Total Correct Answers: Total Correct Answers / (Total Correct Answers + Total Incorrect Answers + Total Unanswered Questions) * 100% = 17 / (17 + 35 + 70) * 100% ≈ 14%.
Total Incorrect Answers: Total Incorrect Answers / (Total Correct Answers + Total Incorrect Answers + Total Unanswered Questions) * 100% = 35 / (17 + 35 + 70) * 100% ≈ 29%
Total Unanswered Questions: Total Unanswered Questions / (Total Correct Answers + Total Incorrect Answers + Total Unanswered Questions) * 100% = 70 / (17 + 35 + 70) * 100% ≈ 57%