Frustration

Danielle gets math time tests twice a week. In theory, reviewing her addition flash cards should help her with passing these tests. Last week, she passed the 1st test on her 1st try and that helped built her confidence. On 2nd test, she got 35 out of 36 so I wasn't too concerned. When I asked her if the teacher reviewed the test with her and she said no. The test got thrown out so the kids can't memorize it and she's not aware of the mistake she made. During our homework time, I make up a mock test with random numbers that adds up to be 21 and gave her 3 min to finish the test. With my test, she had 10 questions left after 3 min was up and she got frustrated and cried because she could not finish the test in time. I told her that my test may be harder because I knew she's able to solve the easy ones. As long as she did her best, I don't care how many times it takes her to pass the test.
Yesterday, she took the 2nd test again and also scored 35 out of 36.
Me: "Was it hard or easy?"
D: " It was easy."
Me: "Did you have extra time at the end to check your work?"
D: "I did check my work and it's all right."
Me: "It's hard for me to guess what we need to work on not knowing what you missed."
D: " May be it's my hand writing because I had to go really fast and I usually take my time to write neatly but not during time test."
Me: "You may be right. Can you write a little neater next time?"
D: "Yeah. I know I can pass test 3 because all the numbers add up to ten"
Me: "I'm sure you will pass this one too once we figure out what concept you've forgotten from 1st grade. "
Me: " Remember you did time test at 1st grade and you went all the way to 19?"

I emailed her teacher to asked for some recommendation on how to help her from home. I know she can do the math; however under pressure, mistakes happens.
Why can't the kids move on when they get 97% correct? The principle may know the answer.