I keep making bad grades on my Mathematics tests. Why?

Yesterday I took a math test on proportions. They are extremely easy but I made a 75%! At the beginning of it, I was psyched about it because I’m good with them. I was excited all day after the test, I thought I was going to get a big fat 105%.
But when I received the test after it was graded, I saw a 75! Why? I’m good at math so I don’t get why I missed 5/20 questions.