Generally speaking, a placement test is designed to measure your academic skills. The point of taking a placement test is not to get the highest placement, but to get the most accurate placement.
Keep in mind doing well in high school can often be entirely different than and doing well in college. Even when classes in high school and college have the same name, the work and skill levels expected will be much higher in college. Please do not be discouraged if your placements are not as high as you expect.
A competency test challenges you to show you have the same skill level as students who complete the coursework for the target area.
A diagnostic test determines if you have the skills necessary to enter a target course.