Occasionally I use my web browser to check and verify definitions in one window while a window is open for a test (multiple choice, Moodle). I wondered WHY I picked such a poor answer on a few previous tests and I just discovered the reason! When you left-click on the window you want to work in, doing so on the Moodle page MAY change your answer selection to correspond to your mouse pointer location. So although you are on say, problem 5, if you click on an area to the right of perhaps problem 9 on the Moodle page to switch back, and you HAD answer A, the new answer may be switched to D, since your cursor was close to the wording, but certainly not the inside the circle.
Recheck your answers to make sure you don't accidentally change an answer you were not even working on! I suspect I triple, or quadruple check now :-)