mysterious changing answers (SAT practice test)

I am currently using the SAT prep feature in Khan academy, and every time I take a mini timed quiz in the Reading and Writing section of the course, I find that the answers that I had selected during the test itself have been changed after I click the 'submit' button. 

For example, during the test, let's say I select the 'B' answer (the correct answer) for question 2. Then, once I click the 'submit' button to finish the test and review my answers, I see that question 2 has been marked wrong, so I assume that I just got the answer wrong and 'B' was not the correct answer choice. However, when I click on the question to see what the correct answer really was, I see that 'B' IS the correct answer, but has not been counted as the answer that I originally selected. Instead, the website will say that I selected the 'D' answer instead. 

This is really frustrating and it has happened for the past 3 times I have tried to take the tests. I originally thought that maybe I had just accidentally changed my answers when I was scrolling to the next question, but I just took another test and paid extra attention to my answers and triple checked them before I submitted -- and the same thing happened again.

Help?