Maths in high school focuses more on applying methods (taking a derivative etc.); maths at university (even on a CS course) is more about constructing mathematical models and arguments.
It can almost feel like a completely different subject.
At one point I met a guy who was working on ways for computers to solve math problems. He told me that early college level problems are actually easier to solve than high school problems for his algorithms. The reason is that while college problems have more steps, and are therefore harder for students, you essentially just need to string theorems together. High school level problems are simpler (less steps) but require significantly more intuition.
And that's what I think happened with my grades: I was never good at math, but I had good intuition. And the switch from using intuition to using actual rigorous mathematical thinking killed me. It was like a completely different subject, and it took me years to catch up.
I wouldn't say if was it was different when it comes to applying methods vs constructing models.
I think it is a good thing. There is a big difference between doing well on a test and understanding the subject. You may think you know basic algebra because you can find the correct value for x, but chances are that you don't really know what you are doing. So having a refresher course telling you what you are actually doing when solving an equation, and how it fits in the big picture is valuable.
The thing is: when you relearn these basic subjects, you do it with a wider field of view. For example, when I first learned derivatives, I knew it had was about rate of change, and I knew the formulas, but I didn't quite get the relationship between slopes and derivatives. It came back later, as I was more familiar with things like linear interpolation and how to manipulate curves in general.