Why do I keep getting bad grades from the subjects that I'm very good at
But I take incredibly bad grades, just because of the calculation mistakes I do. It's not because I didn't solve the question or anything, it's just because of the calculation.


Boy that word "just" is dangerous, isn't it?

The unfortunate truth is that you are not good at practical economics, precicely "because of the calculation mistakes that you do".  You don't think those mistakes are important (hence "just"), but you don't get to decide what is and isn't important in your classes/research/work. The people who evaluate you get to decide what's important.

There are many disciplines where the primary goal is to get the right answer, not to demonstrate your "understanding". It doesn't matter whether you grok the theory if you can't actually execute on the fine details. If you didn't get the right answer, you didn't solve the question.

This does not mean that practical economics is less worthwhile than hihg-level mathematics, which cares less about arithmetic skills.  It only means that practical economics is different.  Math emphasizes different details that  you're more comfortable with and that other people would find boring and/or baffling.


  I can't change this


Of course you can.  You can do exactly what everyone else does to become good at anything uncomfortable: Put on your grownup pants and practice, fail, listen, practice, practice, fail, listen, practice, practice, practice.